"Can't we just call it a satchel?" he says sheepishly, knowing how such an item might be ridiculed back home. "I just came from a massage and it's got all my stuff in … no wait, that sounds worse. I'm building up for a movie and my legs are like lumps of meat and they are killing me, so after the gym I had to have a massage on my thighs."
The gym, admittedly, is where he has spent much of the past decade ever since his role as King Leonidas in 300 revealed a fine torso, leaving female fans longing for more. Producers took note, casting him in a string of romantic comedies such as The Ugly Truth, PS I Love You or The Bounty Hunter. Today Butler will argue he enjoyed every single one of them, although his more recent career decisions speak to a desire to break out of the mould.
Case in point being animated children's movie How To Train Your Dragon, in which he voiced a Viking named Stoick. The 2010 original was a box office hit and the sequel sees his character given a second chance to re-connect with a long lost family member.
The filmmakers could not have known how deeply this theme would resonate. "I have a very similar story. I didn't see my father for 14 years, I didn't even know he was alive," says the actor who was born in Paisley, the youngest of three children of Margaret and Edward Butler. Moving to Canada shortly after he was born, he was 18 months old when his mother and siblings returned to Scotland after his parents' marriage broke down.
"I hadn't heard from him from years. And one night I came home and my stepfather said, 'Keep your jacket on, you're going back out, your dad is in town.' Within an hour, I was sitting in front of my father, who I didn't know was alive until that moment.
"This movie reminded me of that, how when you think that life has gone one way, in one instant, everything changes. You realise it is not what you thought at all and suddenly you are open to a whole world of magical possibilities and romance and heartache," says Butler, whose biological father died six years later from cancer.
"I got to know him but I never really got to know him, like I would if he had been there when I was growing up. It's a tragedy for him not to have known his kids, but also for me not to have known my father."
To this end, he has spent the past 20 years kicking and screaming against anything that might resemble family or settling down. A two-year affair with Romanian lingerie model Madalina Ghenea fizzled out earlier in the year. "I have not met 'the one' yet," says the 44-year-old, adding, "but I hope to be a good dad one day."
Children, it turns out, have been on his mind lately. "It's funny, over the past few days I have had a conversation about kids with three different people, including one of my closest friends. He said something to me last night, which was really moving. He said, 'I wish you'd been my dad.' I thought, 'Wow.' I really respect this guy, he's an amazing individual. But people who used to say, 'I think you'd be a good dad one day,' are now saying, 'Hurry up.'"
Having been linked with Naomi Campbell, Jennifer Aniston and Cameron Diaz, it seems children would not fit into the equation of Butler's playboy lifestyle.
"I genuinely love kids," he says. "When I meditate on the idea of fatherhood, it makes me excited about the prospect of one day being a father and being more settled. From single philanderer to committed steady father!"
But for the time being, he has never met a party he didn't like, popping up at concerts, sports events and festivals around the world. Point in case being Wimbledon last year, where he cheered on Andy Murray.
"It was the first time I had met Andy," says Butler, who attended with his actor friend Bradley Cooper. "The first time we spoke was just after he won the final. I am friends with [Novak] Djokovic and he had invited me to go in his box and I was not going to go to the game because I said, 'I can't go and sit with you guys because, I've got to be honest, I've got to support Andy Murray, even though you're my buddy.' But Novak was amazing. He understood.
"It was one of the most fun days I have ever had. To be there for that moment - the first time a British man has won the singles in 77 years - not just a Brit but a Scotsman - and for it to be Andy Murray. He is a big hero to me. What he has been through and his story is so incredible; you couldn't write a more heartbreaking story. He always stayed the guy he is: a little shy and private."
Spend any time with Butler and you become a participant on a rollercoaster ride of emotions. One moment laughing, the next reflective, he admits he is a mystery even to himself. "There is a part of me that is light and easy and fun but there is another part of me that people do not really see in interviews, which is much deeper and introspective and connected spiritually to something that I feel is way more powerful than myself."
Sober for 15 years, he describes his life as a spiritual quest, recently finding a new sense of peace as a result of taking a year off and discovering meditation. "I love meditating, especially since I moved to Malibu and got a ranch. I meditate in a forest and it's amazing. Every day I get up at 7am and meditate and I swear, when I finish, I am ready to take on the day."
If he does not seem like the Malibu kind of guy, then he agrees. "I am not even on the beach. I am staying in the mountains and I love it. It is like being in Scotland. I would rather be in the mountains than on the beach. I feel so at home surrounded by nature.
"Before I had always lived in Hollywood and my energy is very diffuse. I kind of have that ADD energy, but since I moved to Malibu I understand the value of coming from that more connected place and how much more enjoyable it is to be in a calm place than a crazy, flustered place."
Eager to demonstrate "new Gerry", he fishes around in his bag to find his phone. "Look, my buddy just sent me this text. Now this is a man," he says, going on to recite: "'Protect the helpless, always tell the truth, do good and be upstanding so that God may love you.' He is always quoting me Knights Templar stuff. I dig that stuff."
His quest for spiritual solace has spread to his work. "I'm producing a movie called Headhunters. It's Dear Frankie meets Glengarry Glen Ross, if that's possible," he says of the story of a man headhunted for a job at the same time his son is diagnosed with leukaemia.
"It is about this reconnection between him and his son because his son is basically dying and he realises all the things they never got to do together," he says.
"I think any father-son story is going to be personal to me because of my relationship with my father. But it is going to be personal to anybody because everybody had some kind of relationship with their father and they are always touching something much deeper. I want it to be the smartest, most beautiful, touching movie ever."
He has to leave shortly for the airport, where he will collect his mother and step-father who will house-sit his Malibu home while he spends two months in Australia shooting Gods Of Egypt, an action adventure movie he describes as Avatar meets The Lord Of The Rings. It is also the reason for his gruelling gym schedule. "I am unfortunately playing an Egyptian god. The second you hear the word 'god' you know you're f*****. There is no getting out of the gym."
His assistant tells him he has 10 minutes left if he is going to make it to the airport in time. "I know that in the next three hours I will be lectured about something, and my mum will be putting me in my place," he says with a wink, like he is still that naughty schoolboy from 30 years ago.
Part angel, part devil, he rebelled against authority early on. Head boy at St Mirin's and St Margaret's High in Paisley, he tells this story by way of example. "I think I have always been both. Obviously I had to be a good lad to be made head boy, but then I was sent to the headmaster's office twice, and he was like 'Gerry, you can't be being sent here because you're the head boy and that's not how it's supposed to work.'
"So there was always that rebellious, rogueish behaviour but, at the same time, the teachers knew I was a good kid; it was more that I was over-enthusiastic. But I took the head boy job very seriously and I had to organise all the other school prefects."
Still, his mouth continued to land him in trouble. "One time, I was doing this sixth-year studies class, and there were three of us and the teacher was the angriest man I ever met. He had the reddest face ever. Somebody asked me a question while he was talking, and I turned to answer them and he said 'Butler … you!' I just stared at him and I didn't say anything which made him even angrier. It's a technique I developed with my mother when she was going off."
Butler, nonetheless, would go on to study law at Glasgow University, becoming president of its law society. Working briefly in a practice, he never lost his dream to act, making his film debut in 1997, playing Billy Connolly's younger brother in Mrs Brown.
He reveals how - after years of pummelling his body for roles - it is something of a relief to make an animated movie like How To Train Your Dragon 2.
"You don't have to go to the gym to use your voice. I just have to speak with a little more resonance, and deeper. I wish all my movies were animated," he says, removing various items of clothing to reveal his scars.
"I've had two rhizotomies [surgical procedures to treat chronic back pain] and it's all from training for movies. Right now I have been crushing it again. Your body doesn't heal when you do that for months so, by the end of the movie you are a monster, you're like the walking dead. You do that over 15 movies and stunt after stunt … I have injured myself a lot but you can never take time off because then the whole thing shuts down. So unless you lose a leg, you've got to go back to work so sometimes you're just limping around the place.
"I broke two bones in my neck in Olympus Has Fallen, I almost drowned in Chasing Mavericks, I was in a car crash in another movie." He became briefly addicted to prescription painkillers. "I'm not whining. I love it - it's like getting to play cowboys and Indians as a grown-up, but you do hurt yourself."
Playing a Viking is in his DNA. "That's my culture and that's my history, and every grunt, groan and shriek that you hear, that's in my blood. "
He admires his alter-ego Stoick. "There's a lot to be said for being masculine and strong but, to be able to break away from that and not get too caught up in your ego and your pride, I think that is the sign of a true man. Also, the ability to admit when he is wrong, to be sensitive and vulnerable and yet show great courage and compassion - because that is hard sometimes. It is a lot easier to be a bastard than it is to be good." n
How To Train Your Dragon 2 (PG) is released on Friday.