drupal_goto in new tab
That isn't possible - drupal_goto() happens server-side, and you can only affect tabs/windows with client-side code.

You would need to stop using drupal_goto(), and instead insert some javascript into the page which opens the new URL in a new tab upon refresh.

Some (including myself) would also argue that you have no "right" to do this - let the user decide whether they want a new tab or not. Be careful of trying to force your will onto them. See Target Blank is Evil for a wonderfully opinionated bit of rhetoric on the matter.