The problem with a Gate deck, on top of the mana constrictions of a three-color deck, is that so many of your lands enter the battlefield tapped
Additionally, it's a 7/7 for a cost of just six, which is a fine deal on its own
While Ezio Auditore da Firenze gives all your Assassin spells Freerunning, theyre hardly broken in terms of power level
Because of this, Mystic Forges third ability provides both card filtering (by getting rid of cards we don't need) and card advantage: Any time we find a spell we're happy to cast, it's like having drawn an extra card
We get Gerrards Verdict, Swords to Plowshares, Seal of Cleansing (much more natural of a fit with Bottomless Pit than Naturalize), and even Eternal Dragon
Without it, the deck is really leaning on flying, Spell Queller, or Collected Company being well-positioned for the matchup