Elder Gargaroths ability is triggered by it attacking and blocking, which is where vigilance really elevates this card
As for your collection
This is an excellent card if youre playing a Creature tribal deck, but its one of the best tribal cards without question if youve got a little deck manipulation in there too
The adventure adds something, though not enough to justify spending more mana for the primary effect, which is clunky even at four mana
Commander pre-cons are a much better value toward building a playable collection, and even if they want to buy booster packs, Set Boosters and Collector Boosters contain a lot of cards that arent legal in Standard, such that even if a player were trying to play Standard by buying packs of the appropriate set, they still couldnt just build a deck out of cards they own and know that they could bring it to a Standard tournament, they have to know exactly what to look for to figure out which cards are legal
That ability is "when the last defense counter is removed from this permanent, exile it, then you may cast it transformed without paying its mana cost." This means that if for any reason a battle enters the battlefield without defense counters, it will be put into the graveyard and its controller will not get the chance to exile it
