I have wondered the same for a while now, and I hope someone familiar with Rust can explain it: in many of the benchmarks Go is faster and/or uses less memory despite the GC. Like you, I don't see much trickiness in the Go code.
Another factor could also be that the GC in the Go examples just never runs since it doesn't allocate enough memory. It's hard to say exactly what's happening without tracing the runtime, which would also affect the benchmarks a bit.
It's important to note that while both languages are natively compiled (and I suspect Rust would inch out in that category due to LLVM's backing) most of the overhead would probably be from memory, whether allocations, or cache usage, which makes comparing them in microbenchmarks a little inaccurate.
This is an allocator feature independent of garbage collection; jemalloc does this for example. GCd languages have the potential to do this better since they can do additional analysis whilst tracing for little additional cost, but non-GCd languages can still benefit from this.