1. GCC has more backends than LLVM.
2. Competition is good in general.
3. I expect this will trigger inconsistencies between GCC and rustc; because Rust doesn't really have a specification. Which will force both parties to discuss and solve them.
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.
More likely that GCC has to follow all the bugs and quirks of rustc or no people will use GCC for Rust.
Did you mean target platforms? If so, how is this not already addressed by the rustc gcc (Via libgccjit) backend?