The 37-year-old's triumph at Sea Island, Georgia was his first PGA Tour title.
"I had a hot putter today," he told the Golf Channel. "It's been a hard year for me and finally we got it right.
"I was just trying to hit fairways, trying to take it one shot at a time - that old cliche. I hit fairways, I hit greens and my putter saved me."
Gainey shot a course record 10-under-par round of 60 on the Seaside Course.