Oauth2 Refresh token is not available - pega is not able to maintain the refresh token
Regarding the gDrive-Pega integration, we are looking for an alternate solution of "OAuth 2.0 - Offline access" authentication instead of service account authentication.
As per the google API documentation, "OAuth 2.0 - Offline access" can be leveraged using a generic google account / technical account which can be used for the authentication for gDrive-Pega integration and this solution might be used for replacing the service account authorization. Google supports common OAuth 2.0 scenarios such as those for web server, client-side, installed, and limited-input device applications. Please refer to below google api documentation link which describes the OAuth 2.0 - Offline Access mechanism.