> i don't see the reduced development effort, after all, unless the language is only running on webassembly i still need to implement my own GC for other CPUs.

I'd think porting an existing GC to WASM is more effort than using WASM's GC for a GC'd language?

Writing a GC that performs well often involves making decisions that are tightly coupled to the processor architecture and operating system as well as the language implementation's memory representations for objects. Using a GC that is already present can solve that problem.

Aside from code size the primary benefit on the Web is that the GC provided to wasm is the same one for the outer JavaScript engine, so an object from wasm can stay alive get collected based on whether JS keeps references to it. So it’s not really about providing a GC for a single wasm module (program), its about participating in one cooperatively with other programs.