So, my HP & CP go up when I power-up my Pokemon, which is good to know, but it seems like the only stat used in gym battles is CP. What exactly is HP used for?
3 Answers
HP stands for hit points. It is the amount of health your Pokémon has. Each attack it takes will take away some hit points, and when it reaches 1 or 0 (1 for friendly gyms, 0 for enemy gyms) it will be defeated. So the more hit points your Pokémon has, the more damage it can take in a fight before being defeated.
Well your Pokemon has HP in battle too. The higher it's HP the more hits it can take, both in training and attacking/defending a gym from an opposing team.
If your Pokemon has low HP after you've assigned it to a gym you can build it back up again by using the purple potion bottle in your items. These are collected at pokestops