Hi there, if you wish to use bridge mode, connect an input to channel A, turn the switch on the back of the GT800 to bridge. That's the input side taken care of. The amp will automatically take channel A input, flip its phase 180 degree and apply that signal to the input of channel B. There is no custom wiring involved for the input signal.
The output however requires the use of a 'Y' lead plugged into output A and output B. On Friday night of this week I will put a video up on the media page of the site showing how this works.
Speaker cables don't have earthing. Most will use 2 cores ( PA guys use 4 or 8 core to make setup and tear down quicker ) that connect to the +ve and -ve terminals of the connector on your chosen cabinet. It is fine for a normal speaker cable to lay on the ground ( preferably not beer soaked because it makes your cables and hands sticky ). It is also fine for a speaker cable carrying a bridged signal to lay on the ground, its just carrying a higher voltage.
Cheers
Matt