On any OS which is not hard realtime, there could be arbitrary pauses with any syscall. This is just nitpicking.
More nitpicking: on x86-64, SMI interrupts can cause arbitrary pauses even without any software control involved. Hard realtime on x86-64 is not possible.
Orrrrr, GC pause just means pauses caused by the GC as part of its implementation's work to manage memory.