Question I'm trying to mute the tuner using a MIDI pedal or a momentary external pedal, firstly is this possible

This is possible with both MIDI pedal or a momentary switch. With a momentary switch:
  1. Connect it in the Pedal input, then press the UTILITY button, set the PEDAL type to "Momentary", then go to Calibration parameters in the same menu - the software will ask you to set MAX - press the pedal and hold it until you are asked to set MIN. 
  2. Press MOD key and go to the parameter "Tuner MUTE", set it to PEDAL - that' it.
With a MIDI switch:
  1. Make sure your board switch button sends a latching MIDI CC - it means that it will toggle between 2 values (0 for off and 127 for ON),
  2. Press MOD and go to parameter "TUNER MUTE" , set "Learn" and press ENTER, then press your switch button on the MIDI board, the G-Major will detect the MIDI CC and display it.