I'm not sure what units 5 and 0.1 would be. The two numbers though, should represent inlet pressure (the pressure on the inlet side of the valve, the tank, basically) and outlet pressure. Tanks are usually around 800psi or so when full. The other pressure unit I know of is kPA (kiloPascal) but I think it's something like 1.5 psi per 10 kPA which means 800psi is something like 5500 kPA.
Unless the guage is reading in "thousands of kPA"? I guess that would explain the 0.1 reading though because 18psi is about 100-something kPA which would be 0.1 "thousands of kPA".
There's nothing on the guage itself that says what the units are?