Gargos, Vicious Watcher

Gargos's cost-reduction ability affects only generic mana in a Hydra's cost. For example, if you cast a Hydra spell with mana cost {2}{G}{G}{G}, it will cost {G}{G}{G} to cast, not {G}.

If the target creature is an illegal target when Gargos's last ability tries to resolve, the ability doesn't resolve. If Gargos is no longer on the battlefield, the target creature won't deal or be dealt damage.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Gargos). The mana value of the spell remains unchanged, no matter what the total cost to cast it was.