Google sign-in using Javascript API without triggering popup

Such functionality is not provided through Google API. You should stick with gapi.auth.signIn. I know just one way to make it work, but it's very hacky.

gapi.auth.signIn opens authentication window. Save authentication window url in your app¹. Instead of calling gapi.auth.signIn, redirect user to that url.

To redirect successful authentication back to your website, add/modify redirect_url param in the url². Keep in mind that redirect_uri must be registered in developers console.

Example: https://accounts.google.com/o/oauth2/auth?client_id=1234567890.apps.googleusercontent.com&scope=https%3A%2F%2Fwww.googleapis.com%2Fauth%2Fplus.login&immediate=false&response_type=token&redirect_uri=http://example.com

This way google will redirect user back to your website. access_token is provided through GET params.

¹If google changes their API this may break (since this method bypasses JS API and assumes that all those params in the url will be supported for ever).

²Redirect_url is introduced in offline access flow documentation. I don't think this param was intended to work in any other cases.

I strongly advise not to use this idea (since it bypasses JS API and uses undocumented functionality). Stick with gapi.auth.signIn.

You can the use ux_mode parameter (options are 'redirect' or 'popup') and set a redirect_uri if you want to redirect to a different page.

It's also necessary to authorize the URL for the OAuth client on your google project page.

  function initClient() {
    gapi.client.init({
      apiKey: API_KEY,
      clientId: CLIENT_ID,
      discoveryDocs: DISCOVERY_DOCS,
      scope: SCOPES,
      ux_mode: 'redirect',
      //redirect_uri: custom url to redirect'
    }).then(function () {
      // Listen for sign-in state changes.
      gapi.auth2.getAuthInstance().isSignedIn.listen(updateSigninStatus);

      // Handle the initial sign-in state.
      updateSigninStatus(gapi.auth2.getAuthInstance().isSignedIn.get());
      authorizeButton.onclick = handleAuthClick;
      signoutButton.onclick = handleSignoutClick;
    });
  }