Opting to bat first, Gloucestershire reached 154 for seven in seven wickets. Chasing a victory target of 155, Northamptonshire romped home in 17.4 overs losing just two wickets.
Rob White (62) and David Sales (68 not out) starred for Northamptonshire.
Ganguly and White raised an opening stand of 64 runs before the former India captain fell to spinner Martyn Bell, after scoring 23 that included three hits to the fence.
Earlier, Gloucestershire, who had already qualified as group winners, made 154 for seven with Mark Hardinges hitting an unbeaten 94.
David Brown (10) and Steve Adshead (10) were the other two batsmen who reached double figures as Matt Nicholson took 3-22.
Ganguly went wicketless in his four overs, conceding 28 runs and his only contribution was taking the catch of Ian Harvey off Ben Phillips.
In another match at Riverside, Dinesh Mongia was out for a first ball duck even though his side Leicestershire beat Durham by 79 runs to seal their quarterfinal berth in the Twenty20 Cup.
Batting first, Leicestershire scored 183 for seven with Hylton Ackerman (87) as their top scorer.
In reply, Durahm reached 104 for eight, riding on Jimmy Maher's unbeaten 55 which, however, was not enough to see them through.
It was a forgettable outing for Mongia with the ball also as the Punjab southpaw conceded 23 runs in his three overs without any success.