IF they make a Half-Life 2 movie, I could see it as a prequel, taking place before Half-Life 2 so you don't see Gordon, other then maybe flash backs or pictures.
Maybe how Eli, Kleiner and Barney escaped BM, got to Europe, etc.
Or have it focus on the side characters during the gap between Nova Prospkt and Follow Freeman. Or hell, have take plce on completely different part of the planet without any of the main Characters other then Breen since he is the 'leader' of Earth.
I'm not sure how you would do a Portal movie, again probably a prequel.