There’s no API for programmatically adding users to Authorizer, *but* Authorizer hooks into the WordPress hook edit_user_created_user
to add any newly created WordPress user to the Authorizer approved list.
So I would suggest hooking into gravity forms (somewhere, I don’t know much about it) and creating a new WordPress user there. Here’s an example of how Authorizer creates a local user (your code would be similar):
https://github.com/uhm-coe/authorizer/blob/master/authorizer.php#L811-L821