Is it possible to measure the open tank water level using a pressure gauge
A pressure gauge mounted at a bottom tap on an open-air vessel will read Head pressure, yes. This pressure is dependant on the Specific Gravity of the fluid in the vessel, to convert to level.
Level elevation distance = Head measurement / S.G
Say the level Gauge (or Transmitter) reads 119.78 inches
Specific Gravity of the Fluid = 1.133
119.78/1.133 = 105.72 inches (actual)