According to whom? The staff at Valve pronounce it “Gladys” (or as you put it, “GLADISS”) in the original “Portal” in-game commentary, which I point out in the liner note linked at the bottom of this poem. You can think it’s pronounced however you like, but the people who made the game and created the character outrank you.
The liner notes are the back story for all these geeky poems. They're like a crib sheet, but without the stigma. If someone stigmatizes you for reading my liner notes, let me know and I will take care of it.