You should do the following:
In addition to Google.Apis.Auth NuGet package you should install the following page: https://www.nuget.org/packages/Google.Apis.Oauth2.v2
Add Google.Apis.Oauth2.v2.Oauth2Service.Scope.UserinfoProfile and also Google.Apis.Oauth2.v2.Oauth2Service.Scope.UserinfoEmail to the scopes list (When you initialize the AppFlowMetadata).
Now, add the following code:
if (result.Credential != null) { var oauthSerivce = new Google.Apis.Oauth2.v2.Oauth2Service( new BaseClientService.Initializer() { HttpClientInitializer = credential, ApplicationName = "OAuth 2.0 Sample", }); var userInfo = await oauthSerivce.Userinfo.Get().ExecuteAsync(); // You can use userInfo.Email, Gender, FamilyName, ... }