The 26-year-old forward scored his goal in the 22nd minute after Shafizi Iqmal Khirudin opener in the 20th minute.
Fakhrul Zaman scored Kelantan United third goal in the 45+3 minute after Michael Ijezie scored the only goal for Kuching FA in the 28th minute of the game.
Gassama guided his team to promotion into the Malaysian Second Division League from theM2 Liga (Third Division) two seasons ago.
The win moved Kelantan United to seventh position with 26 points, one point behind Kuching FA who sits fifth position with 27 points after twenty matches.