When using C++ modules with g++ (with flags like: -O1, -O3, etc...), Do optimization flags affect the .gcm intermediate file, or only affect the .o object files?

Yes, optimization flags affecting in direct way .o files and indirectly .gcm

• Compiler directly uses optimization flags, how aggressively it will optimize the code for .o files.

• Compiler's influence on .gcm file is indirect. If it's created with '-fprofile-generate' flag, then compiler can influence the aggression of the program. Data created in .gcda file, will create .gcm file. In other words, compiler can optimize data in .gcda, that creates .gcm file.