A lot of those sites will let you just reset password, send an email to the Google account's associated gmail, and then you're done. I suspect that it isn't thought through as an official feature in such cases, but it does often work.

Additionally, if you signed up with an email and password, you can often just sign in via SSO with a matching email address for convenience and still have the underlying password remain working. Again, possibly not an intentional feature (but rather something like using the email reported via SSO or via email verification as a primary key).