Batsman Kiran Carlson was delighted to carry on from day one, extending his overnight score from 137 to 191 before he was eventually caught at short mid-wicket attempting to become the youngest Glamorgan player to register a double hundred.
Carlson combined with all-rounder Ruaidrhi Smith (38) to put on 97 runs for the 8th wicket before Glamorgan were eventually bowled out for 442.
"The pitch really flattened out overnight, I managed to tick over nicely with Ruaidhri Smith and made a pretty good partnership,” said Carlson.
“Running out of partners is always going to happen if you bat for a long time, though Ruaidhri batted really well.
"It was a career-best at any level. Physically you're prepared with all the fitness work we do, mentally it was tougher trying not to let your ego take over and stay within yourself.
In reply, Gloucestershire reached 161/1 before rain brought play to an early close, with Benny Howell 96 not out.
"They've batted well, but we haven't bowled too badly and it's the type of pitch where as soon as you make that one breakthrough, people do fall quickly,” added Carlson.
LISTEN// Carlson chats with BBC Sport Wales