Oh, I think I know what the issue is now. This bit in the help center is outdated:
Since Integromat has changed to Make, the redirect URLs for your app need to be updated.
Here are all the known redirect URIs you need for your Google Cloud Console OAuth app:
https://www.make.com/oauth/cb/google-custom
https://www.make.com/oauth/cb/google-restricted
https://www.make.com/oauth/cb/google-cloud-speech
https://www.integromat.com/oauth/cb/google-custom
https://www.integromat.com/oauth/cb/google-restricted
https://www.integromat.com/oauth/cb/google-cloud-speech
Once you’ve set these up, you can use/reuse the same Make connection for all the supported Google apps & modules on Make - you’ll only have to enable the respective APIs for your custom app in Google Cloud.
Hope this helps!