You got it! I have a leak coming from 'somewhere' around the gauge. At least that's was my mates exact words as he handed the now leaking/previously not leaking tank back to me.
You don't need to remove the reg to do that, you just need to strip the reg, and to do that you need the flange allen key. The leak from somewhere around the gauge will most probably be from the over pressure vent hole located behind the gauge.