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