Gloucestershire batted first at Bristol and Rikki Clarke who came in after a good game Warwicks second X1 at Binley Road last week to replace Chris Woakes, struck twice early on.
Clarke took the wicket of Spearman for two and then had Porterfield caught by Ant Botha to leave Gloucestershire 29-2.
Hamish Marshall and Chris Taylor picked up the run rate before the latter was stumped to give Stafan Piolet his first wicket of the match. Game poacher Piolet turned catcher to remove Marshall and leave Gloucestershire on 63-4 in the ninth over.
Franklin went to a cracking ball from Botha in a fifteen run 11th over. The sixth wicket came with another catch to give Ant Botha his third wicket of the game and his sixth in Twenty/20 cricket this season.
Gloucestershire piled on the runs and added 42 for the seventh wicket when Dawson was caught by Jim Troughton off the bowling of Clarke for 17.
Jeetan Petal took the wicket of Lewis in the first ball of the final over. One run was scored and then Jon Trott superbly caught David Brown (19). The next ball removed Bannerjee and Glocuestershire Gladiators were all out for 141.
Big hitter Neil Carter only lasted four balls when Warwicks batted. With nineteen runs on the board, Carter was out for five when he was caught Spearman bowled Stephen Kirby.
In poor light, Jon Trott and Jim Troughton started to find the boundary. They had taken the score on to 65 by the tenth over before Troughton was caught by Brown for the second wicket and the end of a partnership of 46.
Ian Bell, who has been called up for The Ashes training camp, came in and smacked a terrific six at the start of his innings whilst Trott made his half century.
Bell hit two more sixes in a knock of 42 before he was caught with Warwicks needing thirteen to win thanks to six consecutive double figure scoring overs.
Trott hit a four to finish on 73 not out as Warwicks won by seven wickets with 23 balls left.
Warwicks are at home to Glamorgan tomorrow. Play is due to start at Edgbaston at 5.30pm and commentary is on BBC Coventry & Warwickshire.