Does this mean that for printing I need to render with GC off (but with the input GC on?) I've noticed that my printouts don't look the same as on screen.

Quote - Macs have hardware gamma correction so they have no need for software gamma correction. NVidia cards have a "sort of" gamma correction because the card doesn't actually sync gamma with the monitor.

 

The purpose of gamma correction is to get the proper accuracy of brightness and color ratios. For PC monitor viewing the GC is 2.2, but for printing (if you do have a high-end color printer) the GC is 1.0. Usually washed-out renders are due to IDL set up too high. Turn it down or, in-between buildings or indoors, turn it off. I rarely use IDL lights.