Updated January 2022
Please note Google has changed the behavior for Google Sign In for the gmail.com domain. Users are no longer able to sign in via the “pop up” browser. Based on testing, custom domains are no yet impacted. We recommend using another sign in provider if you intend for gmail.com users to sign in.
Your users will see an “Error 403: disallowed_user agent”
From this google document, Google has effectively deprecated Google Sign In for captive portals, or at least have made it more difficult to use. So far this change only appears to affect users signing in with their “gmail.com” email addresses. For corporate domains leveraging Google, those still appear to work,
If your computer network intercepts network requests, redirecting to a web portal supporting authorization with a Google Account, your web content could be displayed in an embedded webview controlled by a captive network assistant. You should provide potential viewers instructions on how to access your network using their default web browser. For more information reference the Google Account Help article Sign in to a Wi-Fi network with your Google Account.
So what can you do? For now, if you intend to allow gmail.com users to sign in with google, there is not much you can do besides using a different social login method.
For more information or questions please email email@example.com.