I believe Raymond is aware of the thread issue.  We discussed it.

If gc.disable() would return the previous state of the gc instead of None and an API to enable based on a passed in bool, both of which are written in C and executed with the GIL (or merely another lock protecting changing the gc state rather than the GIL) held and this Python would works with multiple threads all fighting to control the gc state:

@contextmanager
def gc_disabled():
  previous_enable_state = gc.disable()
  yield
  gc.set_enable(previous_enable_state)

But we don't need to modify gc.disable's return value or add a set_enable() if gc_disabled() itself is not implemented in Python.  That's up to the implementer.