The 1-0 loss to Arsenal's youth side was the Hungarian's first appearance since sustaining an anterior cruciate ligament injury against Queens Park Rangers in January.
The midfielder wrote on his Facebook page: "I ran onto the pitch with great excitement because I had the chance to play again after a long time. It's true it was just an Under-21 game but at the moment this is a great forward step for me.
"In the first five to 10 minutes I was short of breath but then I could get used to the speed of the game and felt I would be able to run until the following morning.
"Originally I was to play 45 minutes but in the break I begged for another 15 minutes. Today I do my regeneration on a bike."
Gera has made 172 appearances for the Baggies.