Userless Automated server to server Oauth2 2 legged authentication to Gmail

I've found plenty of information on implementing Oauth2 using a user authorization step, but I'm trying to run a container that automatically scrapes a gmail inbox for attachments transforms them, and exports to prometheus, and I'm having trouble figuring out how to implement this library: https://pkg.go.dev/golang.org/x/oauth2/clientcredentials#Config or any other for that matter to retrieve a token without involving a manual user step.

Will doing this in Go require writing direct API calls since I can't find an existing library to handle this scenario? Would it make more sense to create a Google App password and use generic user/pass SMTP authentication?

First off i understand what you are trying to do.

You have a backend system running in a container which will access a single gmail account and process the emails.

Now you need to understand the limitations of the API you are working with.

There are two types of authorization used to access private user data

1. service account - server to server interaction only works with workspace domains. No authorization popup required.
2. Oauth2 - authorize normal user gmail accounts, requires user interaction to authorize the consent screen

If you do not have a workspace account and this is a normal gmail user then you have no choice you must use Oauth2, which will require that a user authorize the application at least once.

Using Oauth2 you can request offline access and receive a refresh token which you can use to request new access tokens when ever you wish. The catch is that your application will need to be in production and verified, because your refresh token will only work for seven days and then it will expire. To fix this and get a refresh token that does not expire means that your application must in production and verified. This means you need to go though Googles verification process with a restricted gmail scope which requires third party security check and costs between 15k - 75k depending upon your application.

I understand that this is a single user system but that does not mean that you still need to go though verification. When google added the need for application verification they did not take into account single user systems like yours.

Option

Have you considered going directly though the SMPT server instead of using the Gmail api? If you use an apps password you should bypass everything by loging in using the login and the apps password.