Yes you can. The GT8 allows for MIDI program changes to be sent to external devices. All you need do is set the program change number required by your amp into the patches on the GT8 and you are set. There should be a sticky here with the program change numbers required for various Randall amps.
And yes it works great. I do it with my TC Electronics gear and it makes life much easier. Check out Chapter 8 of the GT8 user's manual for a better explanation.
Peace, Joshua