userDelegationWebApp

Manage Gmail Delegates easily and safely - List, Add & Remove delegates

With a web app you can control who has access, and can hide the service account secrets.

This web app controls who is trying to access, and also requires a password.

Preparation - you need to walk through these steps before using the script.

Yeah, don't try to do anything in How to use userDelegation Web App unless you've already walked through the Preparation steps.