[folivier]

Gauges are usually either bottom mount or back mount. Bottom mount will have a square surface for gripping with a wrench above where the threads are for mounting. Back mount will have the same setup but at the rear of the gauge in the middle. You should be able to use a crescent wrench on the flat surfaces but might want to use another wrench on the pipe it mounts into. Rightie tightie, leftie loosey But you probably know this.