Gate Colossus

Once a creature with power 3 or greater has blocked Gate Colossus, changing the power of the blocking creature won't cause Gate Colossus to become unblocked.

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. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.