Gatlin led Sam Houston (9-14, 3-6) with 17 points as the Bearkats ended a four-game losing streak, the program's longest since 2001.
Sam Houston trailed by seven points at halftime, 32-25, and was down by nine early in the second half. Konner Tucker, who scored 14 points for the night, gave the Kats their first lead of the second period with a three-pointer at the 9:23 mark to make the score 46-45. Tucker scored 12 of his 14 points in the second half
Steven Werner, who scored 10 points for the Kats, gave the Kats a 55-54 lead on a follow shot with 34 seconds to play. Elgin Bailey tied the contest 55-55, hitting the front end of a two-shot foul with 20 seconds to player. Tucker grabbed the rebound after Bailey missed the second shot to set up Gatlin's game winner.
Southeastern Louisiana had one last chance to send the game to overtime but Jeremy Campbell lost the ball in the closing seconds.
Southeastern Louisiana (8-13, 2-7) was led by Todd Nelson with 17 points and Roosevelt Johnson with 13 points.
"Give a lot of credit to the kids and how hard they've worked and to my assistant coaches who really came up with some good suggestions on the sidelines tonight," Sam Houston head coach Jason Hooten said. "We've said all along during this stretch that our goal is to get to Katy for the conference tournament. This bunch is capable of getting there and tonight is a step in the right direction.
Four of the Bearkats' six Southland Conference losses have been by a field goal or less with the game-winning basket for their opponents coming either in the closing seconds or overtime.
Tucker and Darius Gatson had big hands in the Bearkat comeback. The pair hit back-to-back three-point shots that cut a 43-35 Southeastern leads to 43-41 at the 10:49 mark before Tucker hit the go-ahead three.
"We've got a great group of young men playing for us this year and I've never doubted them," Hooten said. "This was a great win for them."
Sam Houston State returns to Bernard G. Johnson Coliseum Wednesday to face the Southland Conference's top team, UT-Arlington, at 7 p.m.