I recently bought an '81 280ZX 2+2 and when the car has warmed up the temp guage sits right on top of the left most line; 120is what it says. If I really push it for a couple of minutes it'll go up to 130 or so; right on top of the second line. It also takes almost 20 minutes of driving to get to operating temp, is that normal, and how do I go about getting the guage to read correctly short of outright replacing it (I've read it's a PITA to replace)?

Sounds like the thermostat has been removed check that first. Then check to make sure your wire going to the sensor on the engine is on good. If it is then try to replace that sensor first it is cheaper and might be the problem but if not then the gauge might then be bad.

I got it sorted. The thermostat was frozen open. With a new one in there it sits right below center of the guage after only a few minutes of driving.