Hi @Davide_Tamanini, welcome to the community!
This error message refers to the application you have registered with Google (which in the case of n8n desktop would have a URL rending with …n8n.cloud).
To allow everyone to use your application, you’d need to publish the application in the Google Cloud console. However, it might be easier to simply add the Google user you are trying to authenticate as as a “test” user which should allow them to authenticate even if the app itself isn’t public. You can do so by heading to https://console.cloud.google.com/apis/credentials/consent and then clicking Add Users in the respective section.
Note that the n8n URLs used in n8n desktop aren’t suitable for production use and can change without notice (meaning you might need to run through the auth process again once the Google token expires). So, if you’re trying to set up a production workflow you might want to consider using n8n cloud or self host n8n on a web server.
Hope this helps!