We've had good luck with this case by SKB, part number 3i-2217-8B-C


This case has perforated foam so you can configure it yourself. For a GT/22, tear out everything but 3 squares on each side. For a GT/16, you just leave 9 squares along the bottom. I only needed to remove foam squares from the top layer (there are 3 layers). In the second layer, I opened up a couple areas for an AC adapter and its cable. 

Customers have also recommended these cases for use as pedalboards, so you can mount expression pedals next to the GT: