In a rain-reduced game of 46 overs per side, a 93-ball unbeaten 101 from Adam Hose and Jim Allenby's 90 off 110 balls helped Somerset post 294-6.
Chasing a revised target of 297, the hosts slipped to 62-4 with Craig Overton (2-39) amongst the wickets.
Jack Taylor's explosive 68 stemmed the flow of wickets, but Gloucestershire fell short and were all out for 215.
Gloucestershire have won only two of their six matches and their hopes of qualifying for the knockout stages are hanging by a thread.
Somerset's total owed much to the fourth-wicket partnership of 123 between Hose and Allenby.
The home side never got going in reply and lost Phil Mustard to the first ball of the second over.
Taylor's 50-ball knock gave Gloucestershire some momentum and contained three sixes and eight fours, but once he fell to Jamie Overton (3-53), the innings fell away.
Max Waller finished with 3-37 as Somerset moved onto 10 points from their six matches.
One-Day Cup South Group leaders Somerset beat Gloucestershire by 81 runs to secure a fifth win in six.