I can guarantee you they won't adapt any of the TV shows, there's no way to do it without doing justice to the actual original material (not only that Sunrise is extremely protective of the Gundam franchise, they won't allow for a shitty adaptation of any of the series). This will either be a completely new, one off story (that may or may not continue past one movie) or they will adapt one of the many light novels out there set in the UC timeline (kind of like how the Thunderbolt manga was adapted into two 4 episode OVA's and one movie. Sorry DX, your hopes of me wanting to end it after a live-action G Gundam are dashed ).
Seriously though this will probably suck. G-Savior (the fact I have to type that makes me want to kill myself) was an atrocity (and sadly one set in the same universe as the original Gundam, only way in the future) and I don't see this one being much better. The only way this movie would even have a chance of being passable would be if they got Tomino to sit on and oversee the production (which I highly doubt will happen). I really have to wonder why Hollywood wants to bastardize anime like this (Ghost in the Shell was a major atrocity and I know BAA will be too). Fuck not even Japan can do a proper live-action adaptation (exception being Blade of the Immortal and the first NANA movie), what makes them think they can.