I'd fill it myself... you're just repressurizing a system that has leaked. You fill up your own tires when they get a little low, don't you? It's no more scary than that.

AAR, remember that freon doesn't get low for no reason. Go ahead and fill it to the proper level if it is low - but keep an eye out for leaks.

Presumably the gauge you got has easy green/blue/yellow/red markings on it so it is easy to tell where you are... Oh, and don't freak out if you see the pressure all the way up into the red - when the compressor is not running the pressure goes way up - so just make sure the compressor is engaged when taking measurements.