When you look inside, you see a nearly empty room. It appears to be a storeroom, with shelved walls. The room is full of creates and boxes, and there is a pair of brooms sitting in the corner near the door, which is cracked open only about an inch(it opens out, into the hall).
You slip into the window, perhaps not as quietly as you intended, but not loudly, either. If you move ahead, Gelvin slips through the window behind you, and then draws his sword.
He motions towards you, and then the door, pointing to the left; then taps his own chest and gestures to the right.