Being on gcc, a long-lived platform, also helps ensure the survival of the language even if development of the current compiler (or LLVM) dies or withers.
I am out of the loop though, so if this is true, that's interesting and a bit weird.
Most contributors eventually moved into OpenJDK after it became available.
GCC folks left it around for a couple of years, because GCJ unit tests exercised parts of the compiler no one else did.
Eventually they decided it wasn't worth that maintenance cost to keep it around only for that purpose.