The GC is tied pretty much to the runtime implementation and is strongly constrained by that. For example the current elisp runtime has conservative stack scanning and does not have write barriers. In particular the barriers are needed to allow a more sophisticated GC algorithm. If your runtime is written with an abstract GC interface in mind, where you can register roots, inform the GC of writes, give the GC some access to the stack roots, then you can easily exchange different GC implementations. Unfortunately this is not the case with the grown runtime of Emacs.