Ok, so on gear your pituitary gland and testes are shut down but on HCG only the pituitary gland?
H - hypothalmus - shutdown, not sending GnRH to the pituitary because there is no need to produce more testosterone - your balls are doing it all by themselves with help from HCG. So ... technically this signalling service is shutdown.
P - pituitary - shutdown, as explained above - no GnRH signal coming in and no LH signal going out because hypothalmus is basically telling it not to do anything.
T - yeah they are running - on HCG.
My personal opinion is - it's nice to be concerned about testicular atrophy ... and that's what drives people to HCG. However, your pituitary can "atrophy" also when it's shutdown from doing it's job. So, you can have huge fertile balls but if your pituitary isn't sending LH to them when you go off HCG - your balls are worthless like a 68 Mustang with no gas in the tank - it ain't going nowhere.
The only way I'd recommend HCG is ON CYCLE to retain testicular mass. You have nothing to lose there - as on cycle, all components of the HPTA are shutdown anyway - HCG brings one of those components (the testes) back to life so to speak. People USED to use HCG in PCT - but they realized that doing so simply delays HPTA recovery.