Nimrod's pointers to garbage collected memory are separate from the pointers you can allocate yourself. So you could do manual memory management on the parts that really need it - say for a game in a tight loop - and leave the collector to worry about the smaller and less time-critical stuff.

Also the GC is only triggered on a memory allocation. It doesn't run in a background thread or anything like that. So if the GC fails, then the allocation of memory fails (as I understand it) which means your scenario would be caught early.