Usually, all you have to do is reinitialize the doors and liftgate. It's not so much resetting the GEM module, it's teaching the liftgate and door modules what position the doors are in.
Make sure the liftgate and both sliding doors are latched fully closed, and the fuel filler door is closed. With the key on and engine off, and making sure the power liftgate/sliding doors are on in the overhead console, hit both buttons for the power sliding doors together at the same time. They should unlatch and open, and if they do, after they're fully open, hit both buttons again at the same time to close them. If the sliding doors operate correctly, the power liftgate should come online as well.