@Sophia:3, Welcome to the Forum!!!
I'm sorry to hear you got caught in the Developer's hideous death trap. There's nothing quite as panic inducing as running around that caged room while an endless stream of sewer golems chase you. It's a horrible way to die. It's even worse than getting locked in a room with a troll.
But, relax. The forum is here to help. As concisely stated by the Developer above, there are five levers for that gate. Two of which aren't in the poison area. I know, the Developer is a tricky, son-of-a-swamp-donkey.
This exact issue came up
here if you want to use out-of-game knowledge.
For background, David the Developer is an old-school DM. He thinks killing your beloved characters is a good way to pass the time. I hear he's made killing off Iron Man characters into some type drinking game involving way too much Friguld rum. He also uses kittens to search for bear traps outside orc caves. David also thinks Thanos was right.
Good luck on the rest of the Sewer of Horrors. Just keep in mind, you were warned even squads of Battle Mages were fearful to go deeper.