Let's Play Digimon World Part 12 (How to upgrade your Green Gym)

There's other way to upgrade your gym training after recruiting kabuterimon and kuwagamon, also after buying Training Manual from Piximon. When your digimon in fresh or in training form, if you train specific stats (around 10 times), the gym will get upgrade permanently for that specific stat training. I manage to upgrade HP and Offense station in this video