GCC 13 was added to the compiler packages available in #OracleSolaris 11.4.63, alongside GCC versions 7, 10, 11, & 12, and LLVM/clang 13.
GCC 10 is slated for removal soon, in an upcoming SRU. Users of the gccgo compiler need to note that the removal of GCC 10 will also include the removal of the libgo.so.16 library used by Go programs compiled with GCC 10, and that they thus will need to recompile any such programs with a newer version of gccgo before upgrading to the SRU that removes it.