Setting up Google for use as an Identity Provider (IdP)
Once you have a B2C tenant created you will need to have Identity Providers (IdP) configured before you're able to create necessary policies for signing up and in. As the name suggests these are the entities that will provide identiies, like AD/AAD itself would be a in a non-B2C setup. The list of providers is limited at the moment, but will be added to in the future.
This part of the guide covers how to use Google as your IdP.
Navigate to the Google Developers Console: https://console.developers.google.com and create a new project.
Next select "Web application" as the type of app.
Enter https://login.microsoftonline.com in the "Authorized redirect URIs" field.
You will need to add an additional redirect URI as well, which requires navigating back to the properties of the credential you just created.
This URI should be https://login.microsoftonline.com/te/contosob2c.onmicrosoft.com/oauth2/authresp where contosob2c should be replaced with your tenant's name.
With this in place you are ready to create policies in the Azure portal.
For more information refer to the Oauth 2.0 implementation on the Google side: