If the pool has been clean and clear, it takes a very long time to build up "back-washable" gunk in the filter--I usually only backwash my pool 3-4 times a year, unless I have an algae bloom or something, so that pressure of 18 psi may very well be a good "clean" pressure.

When you turn the pump off, does the gauge go to zero? And then does it consistently come back up to the same number when you turn the pump on? If so, then the gauge would appear to work and I would think your pressure reading is fine. I'm sure some of the other DE filter folks will chime in with opinions (I have a sand filter) but I think it's okay.