Take the gauge off, clean its thread and inside the tube as much as you can with a cotton bud and SCG mix but don't fill the gauge with the cleaner.
I fill the hose with cleaner mix (its about 10:1 I think) let it soak in a sink, then swirl around a lot then flush and flush.
Then blow through with "clean Nx quality" air. I pull apart all the fittings and clean as well. All O rings, vent valves etc and clean them all then dry with clean air. You are trying to do two things, remove all sources of oil but also remove any material in the hose such as steel particles from manufacture.
I assume you might be decanting or equalising Nx mixes? The rule is 40% or below no prob, above its should technically be O2 clean. I would not have a problem with doing this up to about 60%, above that I personally would use a true O2 quality hose and fittings. stainless fittings, and O2 quality hose rather than a cheap transfill whip.
In saying that I know loads of people who use a normal air transfill whip for this purpose. I guess it just depends on how anal you want to be.