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.