Those are 3 orthogonal(ish) things.

Realtime just means that actions triggered by event occur in an explicitly bounded time. It doesn't usually mean that they occur in the same time. So a GC with bounded pause could be used in a realtime system assuming you could guarantee the explicit bounds.

I can't comment on whether or not this is true of Nim and pause time probably isn't sufficient (depends on how it's defined) but they're not mutually exclusive.

And most kernels don't need to be realtime unless they are realtime kernels.

Having said that, of course, the vast majority of GC systems are most definitely not realtime, or anywhere close. But that's just actually true, not theoretically true.