Can a login/ authentication page for a Google app engine (python) web application be created, without using the Google identity platform (but possibly a platform common for users having emails on other domains)? Note: Other email address domains not other social media applications.
A Google Account doesn't require a @gmail.com domain. Other email domains are accepted as mentioned here. When you access to the "Google account signup page", click on "Use my current email address instead" to use it.
You can implement another authentication platform, as Alexander Trakhimenok mentioned, but I don't understand how can this help you.