Gallagher shot a final-rouned 7-under 64 and wound up in a playoff when Johnson, playing in the final group, made a 4-footer for birdie at the par-5 18th.
I just knew I had to make birdie on 18, and then I was like, All right, I got a chance here, Johnson said. Honestly, that whole back nine, I didnt think I was going to catch him.
The players headed back to the 18th tee, and each reached the green in regulation. Gallagher rolled his 6-footer for birdie by the hole, and Johnson knocked his in from 5 feet for his first Nationwide Tour win since 2006.
Gallagher had eight birdies and one bogey to finish at 18-under 266. Johnson, the third-round leader, made six birdies and two bogeys for a 67 in the final round at the TPC Wakefield Plantation course.
I saw him running up the leaderboard, Johnson said. Just to get on 17 and see that I had a chance gave me a little bit of jump in my step.
Johnson took home $94,500 to move up to fourth on the Nationwide Tour money list. It was his fifth career victory on the Nationwide Tour, which he has played on and off since 1990.