Tom Westley (30) was the only wicket to fall for the hosts, who were 69 for one at the interval and looking to put the pressure on leaders Northamptonshire.
Glamorgan are into day two of their clash with Leicestershire, with Jim Allenby posting a century in their lunchtime total of 381 for six.
Resuming on 256 for five at Grace Road, Mark Wallace was soon heading back to the pavilion for 16 thanks to the bowling of Ollie Freckingham - his third wicket of the match.
But Allenby joined forces with Graham Wagg in an unbeaten seventh-wicket stand of 103, with the former on 104 and the latter reaching 51 at the lunch interval.
Gloucestershire were 85 for three on day one of their meeting with Worcestershire at Worcester.
Alan Richardson did the damage with the ball, taking all three wickets for just 17 runs.
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- Jim Allenby