MMTk.io is a library that provides VM-agnostic GC bindings, and several different algorithms you can plug in as a collector (stop-the-world mark-and-sweep, semispace, generational gc, etc)
VMKit was a very interesting project. Do you know why it was stopped? As an alternative I switched to the Mono runtime which works very well for my Oberon+ implementation.
MPS is impressive, but also rather complex; I considered it for my Oberon+ C backend runtime, but then moved towards automatic reference counting instead, and currently add weak pointers to the language, which is as simple as one can get.
If you know of other GCs-as-a-library, please share!