GCC 15.0 supports it with -fsearch-include-path bits/std.cc.

gcc -std=c++20 -fmodules -fsearch-include-path bits/std.cc file.cpp

That will take a few seconds, once it’s done you'll have a cache file at gcm.cache/std.gcm. You’ll then be able to compile faster by removing -fsearch-include-path bits/std.cc from your command.

gcc -std=c++20 -fmodules file.cpp

References:

• Commit adding std module
• Commit adding -fsearch-include-path
• Commit renaming -fmodules-ts to -fmodules