Gym DLC first impressions and review:
The store page warns players that a build with considerable sexual experience is a prerequisite to fully enjoying the experience. I can see this is the case, since a build with little sexual experience cannot do much more than do the job properly and get the XP boost. If the player doesn't do anything lewd with this side-job, it has a convenient side effect of slightly lowering libido for a negligible increase in fatigue. There's no coin to be had from this side-job, unlike the waitress and stripper jobs. Simply accepting the side job with the edict means one less room to fight in when level 3 riots.
The mechanics are simple: Move over to a prisoner, observe their form and then give advice. Observe once more to see what kind of status they are in, once they hit peak condition (red text), move on to someone else. Prisoners will stop working out once their HP hits zero, but players won't know where it is, only a rough guide with a short phrase on how tired the prisoner looks. Wimpy prisoners who reach a personal best and finish their regimen at peak condition will most likely give a bonus to gym rating.
If players are doing this properly, just moving along the prisoners and doing the rounds is enough. Buff prisoners who would be dangerous in physical battles don't need much guidance, the wimpy ones who would be the first to fall in physical combat need more attention.
For sex scenes, I presume players need to get the prisoners hard, take their trousers off and then have their way with them once the appropriate desires hit the thresholds. Sadist players get a different title once they figure out to blueball the target.
For each point in gym rating, enemy stats increase in exchange for Karryn receiving a higher XP modifier. Another reason this job is great for sex builds since they don't care how buff the prisoners are since they're not being engaged in physical combat. For everyone else, there ARE benefits from doing the job cleanly in terms of titles (+1 max warden level for doing enough shifts the most notable one, +1 strength for high gym rating) but it's risky when the prison hasn't been completely subjugated on non-sex builds. Best compromise is to go for Happy End and then unlock the gym titles post-game (doing it post-game with revolts disabled by the appropriate edict is certainly an option, but it's not for everyone).