GC has little to do with FFI though. In C# P/Invoke, you basically just declare a static method as external and specify the library where it lives, and that's that. So far as I know, Java has a similar story these days, no need to write JNI wrappers by hand etc. Go is special not because of GC, but because of its green threads.
GC and FFI do have a lot to do w/ each-other though, because FFI usually introduces unmanaged objects.
When a lot of what you're doing is interacting with low-level platform APIs, you end up having a lot of those unmanaged objects. After a certain point, the upsides of using a GC kind of disappear because you still have a lot of places you have to worry about those objects.
Of course, this can be worked around by providing managed wrappers around those unmanaged object, but at a certain point, it becomes easier to just drop down to an unmanaged but safe language (like Rust) that model the unmanaged resources more accurately. In my experience, it's somewhat easier to provide Safe Rust wrappers around your average Windows API than it is to provide a C# Managed Wrapper around the same.
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And yes, go is special because it has very smol stacks, so doing any kind of FFI on it is a bad idea.
Unless your code is strictly glue between those lower-level APIs, GC is still a benefit for the majority of it. And, on the other hand, the lack of it in FFI is, at worst, similar to C... except you still have all the other language features (like, say, null safety or pattern matching) at your disposal.
I'm genuinely curious as to what would make FFI in Rust easier than in C#, assuming an apples-to-apples comparison (i.e. use of "unsafe" and associated features in both cases"). Most complexity in P/Invoke shows up when you try to use it to automatically map data to managed data types; but in modern C#, you might as well just use raw pointers, stackalloc, spans etc.
Rust seems to excel in having an ecosystem that provides binding helper libraries like PyO3, Neon and napi-rs, Helix, etc. which both ensure "if it compiles, you're doing the C glue properly" and provide a layer for automatic type conversion.
...so requiring `use of "unsafe"` is often akin to forbidding Cargo/pip/NPM/etc. and then calling the language un-productive or faulting a procedural/imperative language for performing badly when you code as if you're writing Haskell.
