Do you find a certain type of training to require more willpower than others (endurance, hypertrophy, strength building, fat loss, etc.)?
I would have to imagine the more challenging and intense the training, the more will power it would take to continue doing it.
Will power is most useful in the gym when training for strength. You have to defy logic when it comes time to beat a personal record lift. Lifting more than your perceived 100% max is daunting the first couple of times. Having the will power to repeatedly push your own limits is an invaluable ability.
Most praise coaches that can make their athletes run through a wall, but if you can do that to yourself, even better.