https://github.com/D-Programming-Language/druntime/blob/master/src/gc/stats.d And neither does any of the callable GC functions. https://dlang.org/phobos/core_memory.html
You can't call any of them directly, and the version of the runtime that's included in releases doesn't have profiling enabled in the GC, which means it doesn't use GCStats. This means the first step would be to clean up the profiling mechanisms in the GC into an API that's easy to work with.
https://dlang.org/phobos/core_memory.html#.GC No longer the case as of 2021/03