How to get FirstName + LastName from the user's google account authenticated using Asp.Identity in MVC5 application

You can get this inside the ExternalLoginCallback method in AccountController (as you are using out of the box project template) as explained below.

    [AllowAnonymous]
    public async Task<ActionResult> ExternalLoginCallback(string returnUrl)
    {
        var loginInfo = await AuthenticationManager.GetExternalLoginInfoAsync();
        if (loginInfo == null)
        {
            return RedirectToAction("Login");
        }

        if (loginInfo.Login.LoginProvider == "Google")
        {
                var externalIdentity = AuthenticationManager.GetExternalIdentityAsync(DefaultAuthenticationTypes.ExternalCookie);
                var emailClaim = externalIdentity.Result.Claims.FirstOrDefault(c => c.Type == ClaimTypes.Email);
                var lastNameClaim = externalIdentity.Result.Claims.FirstOrDefault(c => c.Type == ClaimTypes.Surname);
                var givenNameClaim = externalIdentity.Result.Claims.FirstOrDefault(c => c.Type == ClaimTypes.GivenName);

                var email = emailClaim.Value;
                var firstName = givenNameClaim.Value;
                var lastname = lastNameClaim.Value;
        }

     }