Have you been naughty already? If so, no you cannot join. GDI only accepts virgins, if you have gone around making attacks or spyops, you are excluded form GDI for the set if you didn't join in the 1st place.
Always join GDI immediately before leaving protection, cause otherwise you might forget about it and made an attack: once you attacked you can't join GDI for the whole set.
You can get GDI for free after you join it first by using the bonus points. But as mentioned above, you first have to join GDI first, and to do that it must be before you make any attacks or spy-ops. Once you have joined GDI, you can't leave it for the remainder of the set.
