I went to the marketplace to DL it and got charged 800 points after having bought the season pass a couple weeks ago. I had to call them to get my points refunded. Maybe it's a glitch - IDK. Try the in-game route.
Guys this is what you need to do ...First delete the Market place Data file from your system items in your memory ,then clear your system's cache from the device options, delete the season pass for gears of war 3,then delete and recover your gamertag,download
your season pass once again from your download history and then run the game the map pack should be available now for free from the title menu of the game(marketplace)