Nasty trio of Charm options, and one of the key things to add to multiplayer decks is utility and with three options, all of which are strong, this is heavily worth the spot in your 100 size Commander deck
However Zoodiac is the primary target for the limit on terrortop
destroy them, and if you destroyed a face-up Fiend monster(s) by this effect, this card gains ATK equal to half the Fiend monster(s)'s original ATK, until the end of this turn
Plus, if you do the activated ability, that exiled card remains available turn after turn until you activate it again
The crucial factor, here, is that Gem-Knight is shockingly good against exactly Droll & Lock Bird, which is seeing Main Deck play at the moment
A two-color deck typically works best for beginners exploring multiple play styles