You wont see any spaces as they are invisible :)

Just edit the GAB_admin.php and try to highlight anything after ?>

Chances are that you will grab some space. Delete it. This is most likely the culprit and rare occurence when uploading php files.

there is nothing after it

Try redownloading the package and re-upload the GAB_admin.php

redownloading and using the new file did not work, any other ideas?

got me mate. That does the trick 99% of the time with file uploads.
Can you browse to the page direct? http://www.YOURSITE.com/forum/admincp/GAB_admin.php

If the page is blank doesn't that mean the templates weren't installed correctly?

Usually - but no templates are used in an admnicp panel. At least none that this hack uses.

When I do that I get a white screen... no errors

It does ask me for login/password before it goes white

? this is getting stranger by the minute ?

All of your other admincp page are cool?

yup... everything else on the entire forum... regular and admin is fine