I think it’s air - they’re filled with Glycerini'm not sure why is there water in your pressure gauge.. i sure dont see water in mine.. unless it's just me?
It reduces vibrations on the mechanism and should last longer. Mcmaster sells gauges for ~$10, much cheaper than other aquarium related stores. They are only warrantied for 1 yr, so knowing where to get replacements is valuableafter looking up, TIL there's liquid filled gauge.