I’m not sure I understand the advantage of having a GC in WASM. I wouldn’t expect any languages to use it (because GCs tend to be tightly coupled to a particular language—for example, go’s GC requires careful coordination between the GC and the scheduler—and any sufficiently general GC would presumably give up too much performance).
LLVM has an interface for GCs such that the host language can provide LLVM with information about what kind of code to generate at the boundary between the application and the GC, but that’s just shim code; not a full GC implementation, and even then I’m not sure of any languages that make use of it.
Even still, it’s cool to see the WASM folks trying to tackle these big tricky problems!