Like most electro-pneumatic markers the Ion runs on LP inside.
This is governed by the markers regulator, and the marker is designed to run from HP bottles. Using an LP bottle would be possible but is not recommended by Smart Parts and some believe it may result in a slower recharge time due to the lower source pressure - but doubtfully noticed unless you are at a high fire rate.
That gauge is the LPR Gauge - Low pressure regulator. It measures the pressure INSIDE your markers internal reg. This isn't your tank's output. Your tank's output is on the bottle reg.