You may create your own theme and set it to that GC instance or, you may add some js and css files that change what you need.
When I need tabs for a form, I create a field that it's not on the database and set the tabs code there. You should be able to reorganize everything with jQuery.
Some quick tips:
1- Keep everything within the <form> tag.
2- Add the tab field at the end of the fields list.
3- Make the jQuery code that sorts your inputs to run JUST AFTER the tab html code instead of inside a $(document).ready(). Otherwise you may need to re-set any datetime fields you have, as well as the rich text editors (ckeditor in my case).
If you can't figure how to do it, let me know I think I have an example somewhere but I would recommend you to try to do it yourself first, you may end up with a cleaner way than mine. Otherwise you'll probably just copy-paste :p
Hope it helped!