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.
Maryland point guard Melo Trimble, considered a lock to leave the program before struggling in the second half last season, has withdrawn from the NBA draft and will return for his junior season.
Louisville's Chinanu Onuaku, who recently underwent a medical procedure to correct a heart rhythm issue detected during the combine, will keep his name in the NBA draft pool.
Andrew White III will return to the Nebraska men's basketball team next season after removing his name from the NBA draft.