Glamorgan fought back on day two to take the final nine Gloucestershire wickets for just 145 runs, which included five wickets for 47 to the second new ball after lunch.
Seam bowler Michael Hogan finished with figures of 3/51 and was ably supported by all-rounder Graham Wagg who took 3/59, while Marchant de Lange claimed his 300th wicket when he clean bowled David Payne.
In reply to Gloucestershire's 313 all out, Glamorgan reached 187/4 at the close with Marnus Labuschagne enhancing his position as the leading run-maker in the competition this season with an unbeaten 56 and Nick Selman making 73.
"There is still enough time left in the game to force a decisive result because the pitch is still doing a bit every now and then," said Hogan.
"We bowled a lot better today than yesterday morning and have put ourselves in a decent position. Marnus Labuschagne has been tremendous for us with the bat all season and I believe Australia would be silly not to include him in the Ashes squad because he can also contribute with the ball.
"It has been a decent fightback by the team in this game, which would have been even better had we not lost that last wicket. If we can bat well as the ball gets older tomorrow we can put Gloucestershire under pressure."