If you want to turn the gate on and off within presets, as
@yek suggested, use the GTE (Noise Gate) Block.
Regarding turning off the Global/Pre Gate, this was answered in your thread in the regular FX8 Forum:
"On the FX8, press the <PAGE> button twice to get to the GTE page. Turn the <NAV> knob to select "THRESHOLD", turn the <VALUE> knob counterclockwise until the value reads "OFF".
In FX8 Edit, click on the "Gate/Out" button and select "In/Gate". Set the threshold to -100.0 dB by entering the value in the box above the "Threshold" knob or turning the knob fully counter-clockwise."
The -100 dB setting essentially turns off the gate.