GCC’s language status page says it doesn’t support modules yet.
C++20 support is not complete (which is fair enough given that we’re in 2020! And C++20 technically doesn’t exist yet…).
However, with some flags and a development branch you can play around with the in-progress implementation — read more about it on GCC’s Modules Wiki.
The default language version in GCC 10 is C++14; GCC 11 ups that to C++17.