You can't use Grachomp on any Crown Tundra Pokemon in Series 6. The Series 6 banlist and all new Crown Tundra Pokemon will return to ranked starting November 1.
Edit there is a man in the Battle Tower you need to talk to to give Garcomp the Battle Ready symbol. It will forget all moves and learn the newest 4 level up moves.
Experience and reading the Series 7 ruleset.