Please be as specific as possible when letting us know what is missing or incorrect. For example, say "this game's trophy difficulty should be 8/10", with a link to a website corroborating this difficulty estimate (if possible), not "this game's trophy difficulty is wrong".
Fantasy Strike gems can be used to unlock cosmetic content such as costumes, alternate animations (such as intros, victories, practice routines), and special effects when you get a KO. You won't get a competitive advantage, but you will get a style advantage.