My train from Cardiff arrives into Bristol Parkway at 0930 and leaves for B'ham at 0940 via Cheltenham arriving at 1010 & departs at 1012. I am not sure if the train stops at Gloucester but guess would be easy for you to get on at Cheltenham.
Hoppy if you've not yet bought your train tickets I'm gonna be driving up from Cardiff on Sat if you wanna grab a lift - going up the M50 tho so not going near Gloucester sorry John. Drop me a PM if you wanna lift. If not not worries enjoy beers on the train and enjoy the match