If I use GMAIL to authenticate then can ANYONE use my app? how is this secure?

How can the gmail user list be controlled?   I setup the gmail authentication and it works for the app

 app.UseGoogleAuthentication(new GoogleOAuth2AuthenticationOptions()
            {
                ClientId = "9961223819-cjk4s3e---c7xhk7oe-cll.apps.googleusercontent.com",
                ClientSecret = "AUIvc3I---APAlPxmV5iI"
            });

Now it lets the Google part authenticate a user.  But this should only be for my own Admin but thats impossible because I would have to approve user's use of gmail and my app

is there a way to control an email address list, so only they are allowed to get access?  how can I maintain who gets access?

Will also ask my cat, I can guess the answer, (meow) but at least its something