Are users only able to access the app if they have a google account?
There are a few authentication providers you can choose from
You can read more about access and how to grant it here:
I’ve found that DropBox is a good data source if you want people to be able to sign in using any email they want; because people can sign up for their DropBox account using any email they want.
You also have the option of “Any Provider” which allows the user to make the selection of how they wish to log in; of course they have to pass this along to you so you can grant them access to the app via that email before they can try and login (otherwise they’ll just get rejected).