Can I use the G-Force to switch channels on my amp? Does it have a build-in relay switching system that I could use to switch channels on my amp together with the preset change on the G-Force?
The G-force is not capable of doing any relay switching.
However, there are special boxes on the market that do MIDI to relay switching. One manufacturer would be MIDI Solutions. You can find more info on their website.
The G-Major, G-Major 2 and the G-System are both capable of switching your amp with the built-in relay switcher.