"I suppose it's the Roy of the Rovers thing: if I had to have one moment of my life again, it would be scoring a try for Scotland before finding Mangala," says Sir Bill, who this week announced plans to retire as chairman of Cairn Energy in May after more than 20 years on the board.
Founded by Sir Bill in 1989, Edinburgh-based Cairn was propelled into the big time by a series of bumper finds in India, starting with the Mangala discovery in 2004.
But reminiscing happily about the thrill of playing in front of 70,000 at Murrayfield, Sir Bill reckons there is something particularly special about sporting success.
"Scoring a try at Murrayfield, it's such a collective thrill because whatever anyone's business career is, actually people are much more drawn to people's sporting careers because there's an emotional connection."
The satisfaction of winning five caps on the wing for Scotland from 1977 to 1980 was especially sweet for the young sportsman because he was only in the fifth team in his final year at Fettes College, the Edinburgh private school.
"It just showed that if you have determination and belief what you could achieve."
Sir Bill says the lessons he has learned from sport about teamwork and collaborating with people from a wide range of backgrounds have been "hugely important" to him in business.
The son of Scottish fund management legend Jimmy Gammell, Sir Bill also praises his father for encouraging him to believe in himself. "The difference between being successful and unsuccessful is all in the mind," he reckons.
Sir Bill has won renown for exhorting his troops to take "brave pills" and to be prepared to "pick up the ball and run with it" over the years.
While such talk may sound folksy, he has enjoyed huge success in business after making some pretty big calls himself.
Cairn Energy evolved out of an insurance broking business that Sir Bill acquired when he was 24, after tiring of accountancy training.
Oil and gas may have seemed a natural progression, given the interest Jimmy Gammell developed in the industry at the Ivory & Sime fund management business. This provided backing in the 1950s for an oil business developed by future US president George Bush senior.
As a young boy, Sir Bill struck up a friendship with George Bush junior, who became president too, that has endured to this day.
He also counts ex-prime minister Tony Blair, a former schoolmate at Fettes, among his friends, but hasn't been in touch with him recently.
Cairn Energy cut its teeth in the oil and gas business in Texas and the North Sea. However, the huge success the company enjoyed in India came years after the company decided to plough what was a lonely furrow in South East Asia in the 1990s.
The company made its first big find in Bangladesh in 1996 after acquiring the Holland Sea Search business run by an early champion of Asian exploration, Mike Watts.
Sir Bill and Mr Watts struck up a partnership that became one of the most successful in the Scottish oil and gas business.
Sir Bill is full of praise for Mr Watts, now deputy chief executive at Cairn, who persuaded him to get into India by acquiring the Command Petroleum business in 1996.
"Mike, I saw, was someone with huge vision and ability."
Cairn decided to go big on Rajasthan only after Mr Watts saw an image of a block in the state in Shell's headquarters in Holland while the pair were on a visit to talk to the giant about Bangladesh.
Then, with Sir Bill leading on negotiations with the Indian authorities, Cairn mounted an exploration campaign in Rajasthan that took six years to pay off.
Cairn made the Mangala discovery with the 16th well it had drilled on an under-explored patch of desert in Rajasthan state.
The company had bought Shell's 50% share in the licence concerned in 2002 after the oil and gas giant decided it wasn't worth the risk of drilling on.
Cairn brought the Mangala field onstream in 2009 after defying the convention that said junior oil and gas companies should leave majors to develop fields.
In 2006, Sir Bill was knighted for services to industry.
Sitting in the boardroom in Cairn's office, overlooking Edinburgh Castle, he is pleased by the thought that the finds Cairn has made in Rajasthan will be onstream for years and end up producing around 30% of India's oil.
"The three major fields in Rajasthan have produced $18bn (£10.7bn) in revenue and that's a pretty amazing sum."
As Cairn aims to create then realise value, Sir Bill reckons the company delivered the goods when it paid $3.5bn out to shareholders in 2012 on the back of its success in Rajasthan. He is thought to have got around $8m in respect of his holding.
The payout was funded from the $5.5bn sale of the bulk of the company's holding in its Indian business to Vedanta Resources in 2011. The sale was only approved following months of wrangling in India, which included an intervention by David Cameron on Cairn's behalf.
Cairn paid out £480m in 2007. Not bad for a firm that joined the stock market with a capitalisation of £25m in 1989.
With that in mind, Sir Bill may have been surprised the company provoked an uproar when it planned to award him a £3.5m incentive to seal the stake sale, in the form of a £2.5m share award and a £1m charitable donation.
While Cairn backed down on the share award, it suffered a 67% vote against its directors' remuneration report at its shareholder meeting in 2012 as it proposed a separate £1.4m "termination payment" paid in lieu of notice when Sir Bill moved from being chief executive to chairman in summer 2011. The rangy 61-year-old, however, seems philosophical about the matter, suggesting Cairn could have liaised better with investors about what was proposed.
"It is what it was but I don't have any kind of bitterness or whatever. I believe we achieved something incredible. It wasn't something I asked for; it was proposed, maybe unfortunately, but you move on," says the father of four.
Sir Bill has really enjoyed the role of chairman which he says he took on to ensure a smooth succession to the leadership of Simon Thomson, the former commercial director who succeeded him.
The teamwork advocate speaks very highly of Mr Thomson, who joined Cairn as a young lawyer in 1995 and of Jann Brown, who worked her way up to the key chief financial officer's post.
Sir Bill believes Cairn is very well-placed for the future, with the balanced portfolio developed by Mr Thomson.
This combines potentially transformational exploration in areas like West Africa with less risky activity in the North Sea.
Asked whether he has any concern about the possible implication of a vote for independence for Scotland in the referendum in September, Sir Bill said: "I'm not going on the record," adding: "That's a personal decision for everyone."
Sir Bill says the industry prizes stability, but notes: "The North Sea has had its issues with fiscal stability over the years."
While some may think Cairn Energy's decision to invest $1bn on a drilling campaign off Greenland that has not made any commercial finds may have followed a brave pill too many, Sir Bill says only time will tell.
"I own up on behalf of all the team and say Greenland we ended up spending more than we expected to because we had to have a two-rig strategy from a safety point of view but the group had got a $10bn market cap(italisation) and how do you move the needle?"
He adds: "I don't know what will happen in Greenland in terms of the next few years but my suspicion is someone will find something very significant and the acreage we have got around the Pitu block has got massive potential."
Sir Bill is looking forward to devoting more time to interests that include the Winning Scotland organisation he founded to help young people learn qualities like resilience through sport.
He is excited about the potential of the Genius gluten-free food business that he chairs. A coeliac himself, Sir Bill was an early investor in the business.
Looking back on a long career at Cairn, Sir Bill admits he got it "spectacularly wrong" about Bangladesh in the 1990s when the company found curbs on exports limited the market for its output.
But he concludes: "The only regrets I would ever have, and I don't think I've really had them, is if you don't follow your gut instincts and you're persuaded by your colleagues to take a particular position.
"As the leader you must be brave and you must say I've listened to you, I've reflected and I understand but my judgment is we're going to do this."