gilded arch
After I died in the goo (it was rising 10x faster than I could run), I realized it was framerate related. After I set it to 30fps as maximum in the settings, the goo was slow enough to beat. But once I reached the top - the lamp was back up there again, and I saw the key fall - and I think the lock on the gate is back, as I can't open the gate and escape anymore, and I can't finish the DLC 
