The package sys-devel/gcc when USE=d does not place suitable link(s) in /usr/bin. For example with sys-devel/gcc:11 installed this is the output of "equery f sys-devel/gcc:11 | grep gdc": /usr/share/gcc-data/x86_64-pc-linux-gnu/11.2.0/man/man1/gdc.1 /usr/x86_64-pc-linux-gnu/gcc-bin/11.2.0/gdc /usr/x86_64-pc-linux-gnu/gcc-bin/11.2.0/x86_64-pc-linux-gnu-gdc so one cannot simply run "gdc" or "gdc-11.2.0" to run the GCC D compiler. (Such also applies to the v10 and v9 versions of GCC as well.) Since the GCC ebuild is largely tied to the eclass/toolchain, I imagine this needs to be addressed within such. Can this be rectified?
gcc-config is responsible for creating the symlinks in /usr/bin. It's likely that gcc-config is not called automatically when you change USE flags on sys-devel/gcc. Please try calling gcc-config to re-select your current GCC profile. That should generate the missing symlinks in /usr/bin.