Do I need a new welding guage?

It "might be" the valve packing itself. Acetylene bottles hold pressure in the valve packing, that is, you only open the valve part way during use. But high pressure bottles such as O2 and I think also CO2 the valves are of course packed but have a "back seat." You are supposed to open the valve "all the way" and "snug" it against the backseat just as you would when closing it. This seats a "valve" up in the stem and prevents stem loss.

So with the valve closed, the packing "could" leak. It's also possible hoses might be porous, along with other possibilities mentioned.