He is one of 21 Europeans who are competing in this week's PGA Tour flagship event, with a prize purse of $10m up for grabs. The Scot was also among the eight Europeans out in the morning half of the draw to be under par on the Stadium course.
One of three Scots in the field, Gallacher started out with a bogey on the opening hole but carded a birdie on the fourth. The two-time Dubai Desert Classic winner then bogeyed the par-three eighth hole in finding a greenside bunker but birdied the par-five 11th before chipping in for birdie at the par-five 16th.
Gallacher hit a wedge into the par-three island green 17th for par before making his way down the last and holing an 18-foot putt for a birdie.
However, Ian Stoddard, his manager who had walked nine holes alongside his client, missed out on seeing Gallacher's finish as he was headed towards nearby Jacksonville Airport and a flight home.
"It was great to birdie 16 and 18 so that will make lunch taste a little nicer," said Gallacher. "But I was saying to Damien [Moore, his caddy] when we first got here this course should suit the Europeans and that was the case.
"I really like the layout and a 70 is probably the worst I felt I could have shot today. So even though it got pretty hot out there near the end of my round I felt pretty comfortable.
"It was pretty quiet early on with not that many spectators around but by the turn there were people everywhere. Then coming to 17 it's just amazing how many people there are sitting on that bank."
His compatriots, Russell Knox and Martin Laird, were both out in the afternoon half of the draw. Meanwhile, Russell Hendry of the United States enjoyed the clubhouse lead with a score of seven-under par 65 with eight players, including the English duo of Lee Westwood Justin Rose, well in the mix with scores of 67.
It was both Westwood's and Rose's equal lowest round in the event. However, Westwood might have missed out altogether as a result of an early morning call of nature.
"I had the alarm set for 4.45am but had a call to nature at four o'clock and getting back to bed, I cracked my leg on the bed and I thought 'Oh, no' so I have been struggling with that all day," he said with a smile.
A winner recently in the Malaysian Open, Westwood got his round off to a promising start by landing on the green 18 inches from the hole. "That was a nice start to the day having an easy tap-in and then had a birdie at the next," he said. "That was a great way to start the day."
Spaniards Sergio Garcia and Gonzalo Fernandez-Castano were also on five-under.