Hello Marco,
Thank you for your message.
Could you please elaborate more on the issue you're experiencing? More specifically, are you following any particular documentation in order to implement Google OAuth? If so, please link it in your reply, so that we'd have a better understanding of the situation you've reported.
At the same time, please keep in mind that the issues related to OAuth are out of
scope of the Google Cloud Platform support thus, I'm afraid, that we won't be able to fully assist you with it. However, based on the information you provide (such as the documentation you were following), we may be able to tell you the correct support channel.
Thank you in advance!