This is a very simple tool to determine which Pokemon you should take to the gym.
It was designed for users who want a quick estimate of what Pokemon to bring without having to enter lots of information. It can also be used as a quick and easy Pokedex.
Simply search for the Pokemon you are about to fight, tap calculate and you will see a list of all Pokemon with effective moves.
Pokemon tagged with "Unaffected by" are both unaffected by the opponents attacks and have attacks which are effective on the opponent.
Pokemon tagged with "Effective" have attacks which are effective and are not weak against the opponents attacks.
Pokemon with no tags have effective attacks but may be weak against the opponents attacks.