I've found it's possible to disable the social login providers quite easily (we had this discussion here on the mailinglist recently), but I don't see an option to disable Gravatar. I've created an issue about this a while ago. https://gitlab.com/mailman/django-mailman3/issues/1
I don't need the gravatars at all. If someone feels they really need them, they could just be added as uploads directly to the profiles. That way each user can choose whether or not to display one and the "gravatars" would be consistent between their emails.