Hello Nyugaton, welcome to Using English!
1. It was performed at P. G., the opera house named after the architect, C. G.
2. It was performed at P. G., the opera house named after the architect C. G.
#1 implies that P.G. was the architect who designed the opera house. #2 doesn't imply this. So in this context, since he did design the opera house, I would keep the comma.
All the best,