At the moment, SE uses the "email" field your profile to look up your avatar on Gravatar. This has a few issues:
- If your email on SE doesn't match the email you gave Gravatar (because, for example, you logged in with Google and it defaulted to your gmail address), the Gravatar option will silently fail to appear with no indication anywhere what is wrong, or even that Gravatar is supported.
- If you don't specify an email, it instead uses a hash of your IP, leading to the same problem as above with the additional possibility of your avatar randomly appearing, changing, or disappearing as your IP changes, as documented here.
- If you want to use separate emails - for example, because Gravatar profile pages are completely public, or because you use email filtering and want to easily discriminate between emails from the two sites - you are completely out of luck.
It would be nice if there were a separate "gravatar hash" field that you could use to override the default behaviour of using a hash of your email.