It's supposed to be pronounced "glad-DAHS" (the DAHS rhymes with sauce). Though they never pronounce it in game, in the lyrics of Still Alive, they sing "when I look out there it makes me GLaD I'm not you." And the DOS is based off of (and stands for) the Disk Operating System, which is pronounced "DAHS" (rhymes with sauce). When pronouncing it fast, I suppose one would say glad-DUHS, but it's supposed to be pronounced glad-DAHS. Rhymes with sauce!