Not sure about Gears of War, but a quick search at Amazon shows only 360 for all three versions. As far as Halo goes; Halo: Combat Evolved (first game) and Halo 2 are XBox, everything after Halo 2 is 360 only.
They just released an anniversary re-edition of the first Halo CE game for the 360, my son and I just played the whole thing through and its absolutely awesome, better graphics, sounds and more dialogue. Also extras can be found as well during the game. It was like playing a whole new game in some ways and running it on the latest 360 gave it better campaign mode performance.
It was selling for $39.97 or so at Walmart.