So I have this code 
My question is how do I configure the UserCredential if I\'m already authenticated via OAuth?
The current scenario is the c
We can create GoogleCredential directly from the access token and use it instead of UserCredential as well. I just think it might be helpful for someone. Here is a code snippet of GmailService initialization:
GoogleCredential cred = GoogleCredential.FromAccessToken(accessToken);
GmailService service = new GmailService(new BaseClientService.Initializer {HttpClientInitializer = cred});
                                                                        Assuming you already have the tokens you can do the following
string[] scopes = new string[] {
    PlusService.Scope.PlusLogin,
    PlusService.Scope.UserinfoEmail,
    PlusService.Scope.UserinfoProfile
};
var flow = new GoogleAuthorizationCodeFlow(new GoogleAuthorizationCodeFlow.Initializer
{
    ClientSecrets = new ClientSecrets
    {
        ClientId = "xxx.apps.googleusercontent.com",
        ClientSecret = "xxxx"
    },
    Scopes = scopes,
    DataStore = new FileDataStore("Store")
});
var token = new TokenResponse { 
    AccessToken = "[your_access_token_here]",
    RefreshToken = "[your_refresh_token_here]"
};
var credential = new UserCredential(flow, Environment.UserName, token); 
Then you can pass your credentials to the service's initializer
Reference Google API Client Libraries > .NET > OAuth 2.0