I'm sure I've used this same signal sub-consciously in the past. Fortunately most spammers don't bother to add avatars of any stripe. Although I've also found that even many actual legitimate commenters (and specifically WordPress users) don't know about or don't have a gravatar account either. Better yet, I've turned off traditional comments in the past in preference for webmentions, for which I've yet to see any native spam yet. The majority of people using this method seem to take their identity a bit more seriously which also raises the signal-to-noise threshold.