The Rams (3-9), who trailed 57-28 at halftime two days ago in an 88-73 loss to Connecticut, were sharp early this time, shooting 69 percent (20 of 29) in the first half.
Gaston scored 20 points as Fordham took a 47-31 halftime lead. The senior forward now has 1,514 points, ninth on the school's career list.
The Saints (2-10), in their first game since Dec. 9, rallied to shoot 60 percent (15 of 25) in the second half. They got within 63-62 with 10:04 left after consecutive 3-pointers by Rob Poole and Rakeem Brookins, but three straight Fordham baskets made it 69-62.
Poole had 21 points, Brookins 18 and O.D. Anosike 16 for Siena.
Bryan Smith added 15 points for Fordham.
Duke and Cincinnati, 2016 NCAA tournament teams, highlight the eight-team field for the Naismith Memorial Basketball Hall of Fame Tip-Off Tournament.
NCAA champion Villanova will play Notre Dame and Pittsburgh will play Penn State in the inaugural Never Forget Tribute Classic at Prudential Center on Dec. 10.
Florida guard Brandone Francis-Ramirez has decided to leave the team after two seasons.