The set lever in the GP 3100 is released by pushing on a button as opposed to
releasing a screw. The button is the cylindrical protrusion just to the left of the screw to the left of where the stem goes into the movement.
The good news is that when the stem comes out, it can sometimes be reinstalled without even opening the case. Still not something I'd recommend trying on your own as forcing it or manipulating it inappopriately could conceivably damage something.
Glad to hear it worked out quickly and easily!