Gain comp is primarily for when the rack mixer/Stage box (used with a console) and preamp control stays with the rack. In this instance, the preamps are getting set for monitor levels, but more gain is needed upstream for FOH. You use the gain compensation for that.

Maybe the same in other words:
When an analog input is used in more than one mix then the level of that input in all those mixes changes when the preamp gain of that input is changed. The Series III eco system offers a trick to get around this problem.

Firstly it allows you to assign gain control over (the analog part of) an input to a single mixer channel in the system. Can be on the FOH mixer, can be on the monitor mixer…

Secondly (and preferably after the preamp’s gain is set ballpark right) it allows you to engage Gain Compensation on that mixer channel. This freezes the input’s gain to its second "GC" network stream. Other mixes can use this GC stream and locally trim it (15 or 20dB up/down) where any 'reasonable’ gain changes (15 or 20dB up/down) on the mixer channel ’in control' will not affect these levels. Pretty neat.

Yes, Gain Comp should be engaged after setting Primary input first.

You get a 15 dB +/- swing. After that the inputs share gain.

Is there a way to activate this or "turn it on"? I have my 32r hooked up on stage and set in Monitor Mixer Mode and then a network cable running to my FOH console which is a SL32 and all the channels show up on the console but they are still labeled as preamp gain. Like said above, the gain is loud in the monitor mix channels on the 32r but the FOH console is really low but I can't figure out how to get the gain compensation to show up.