To be fair I don't think this is a blocker for Go to implement GGC and CGC. .NET supports interior pointers too, for example. You just have to design your allocator right to allow the card marking to work.

In practice I think that the biggest problems are compiler related and affect both Go and Rust. Go and Rust both use conservative GC on the stack and as a result they take a lot of shortcuts. For example, LLVM (and GCC as far as I'm aware, though someone like DannyBee may correct me here) loses the distinction between integers and pointers by the time they get to the machine instruction level, which makes a lot of optimizations easier to implement but also making it impossible to generate precise stack maps. Fixing this would be a lot of hard work. It's probably easier in the Plan 9 compilers, of course, though I suspect it's still going to be a lot of hard work.