Golang google client api, how to get the user information with `Userinfo` struct

I am currently building a backend Rest API with Golang to handle HTTP requests from a mobile application. One of the features that I am now implementing is a signup/login by using an external provider, e.g., Google, Apple, etc.

For Google, I've read this article on how to authenticate with a backend server. The main idea is to send a token id to the backend via a POST endpoint and validate the content of the Token. Once the Token is validated, I can retrieve the user information from the backend and create an account (if it does not exist).

So far, with the oath2 Golang package, I can validate the Token like so:

func verifyIdToken(idToken string) error {
	ctx := context.Background()
	oauth2Service, err := oauth2.NewService(ctx, option.WithoutAuthentication())

	if err != nil {
		return err
	}

	tokenInfoCall := oauth2Service.Tokeninfo()
	tokenInfoCall.IdToken(idToken)

	tokenInfo, err := tokenInfoCall.Do()

	if err != nil {
		e, _ := err.(*googleapi.Error)
		return e
	}
	fmt.Println(tokenInfo.Email)
	return nil
}

PLEASE NOTE: To obtain the token Id, I am using the Oauth playground, and I set these scopes:

https://www.googleapis.com/auth/userinfo.email
https://www.googleapis.com/auth/userinfo.profile
opened

After searching on oauth2, I noticed a type UserInfo containing all the info I need. However, the tokenInfo object does not return all the information from the user, such as first name and last name. But, I'm having some difficulty on how to get UserInfo.

In short, I created a function called getUserInfo like so:

func getUserInfo(service *oauth2.Service) (*oauth2.Userinfo, error) {
	userInfoService := oauth2.NewUserinfoV2MeService(service)

	userInfo, err := userInfoService.Get().Do()

	if err != nil {
		e, _ := err.(*googleapi.Error)
		fmt.Println(e.Message)
		return nil, e
	}
	return userInfo, nil
}

NOTE: I called the getUserInfo within the verifyIdToken
userInfo, err := getUserInfo(oauth2Service)

However, I'm always getting this 401 error:

googleapi: Error 401: Request is missing required authentication credential. 
Expected OAuth 2 access token, login cookie or other valid authentication credential. 
See https://developers.google.com/identity/sign-in/web/devconsole-project., 
unauthorized

With that, I'm not sure what to do.

I think you have found the answer to your question for a long time, but I still leave my solution here.

The UserinfoService doesn't actually pass a token inside. To do this, you can use code like this:

import 	"google.golang.org/api/googleapi"

    token := "ya29.a0Aa4xrXMAp..." 
    userInfo, err := userInfoService.Get().Do(googleapi.QueryParameter("access_token", token))
    if err != nil {
        e, _ := err.(*googleapi.Error)
        fmt.Println(e.Message)
        return nil, e
    }