Have the same issue here is a work around. Go to OFPEC and download a program called Eliteness 2.23. Once installed open the Gambit mission.sqm and save over the existing than it should open in the ArmA Editor. Hope this helps.
Here is the link if that helps:
http://www.ofpec.com/ed_depot/index.php?action=details&id=383&game=ArmA