I am developing an app for Google assistant and I need a suggestion about the authentication of users for my app so that i can manage their data according to their unique id. I am using Firebase as the database and I think that authenticating the user with a Gmail account is the best way to go about it. So is this possible? I'm also taking other suggestions.
You have a couple of options here, depending on your needs.
If all you want to do is track repeat visitors to your Action, then you can use the anonymous userId that is available. This isn't really "authentication" and it certainly isn't a user identity, but if you get the same userId from a session, you know it is the same user that visited before with that userId.
If you already have accounts for your system, then going with Account Linking, as @ido-green has suggested is preferable, although significantly more complicated to setup.