There are not ToS associated specifically with gmail. The service has no guarantees or expectations set whatsoever. You get better guarantees on email reception

There are not ToS associated specifically with gmail. The service has no guarantees or expectations set whatsoever. You get better guarantees on email reception from running your own server. Therefore it's hard to say what the gmail user could even base their complaint on. Google can do whatever they want with the incomming email as far as categorization and blocking goes, and nothing in the nonexistent ToS is violated.