My GLD has ver. 1.1, can I upgrade straight to 1.2? Or do I need to go through each upgrade?
You can go straight to 1.20, if you want… But it can be a point to read the release-notes of the former updates, so that you know about all the new functions you are getting 
:
Be aware that you also need to update your iPad app (if you have one) to make it work with the new firmware.
There has been some issues with v1.20 regarding storing or recalling scenes. And I don’t know when A&H will release a fix.
Personally i have not upgraded my mixer yet…
Willy
FOH-tech.
GLD-80
AR2412
AR84