Damage Gate lets you Special Summon a monster from the Graveyard with less ATK than the battle damage you took from an opponent's attack. With that being said, after sending Labyrinth Wall that has 0 ATK to the graveyard, you can special summon it after taking any amount of battle damage.
Declares a Level / Discards for effect / You discard
How is it a -1? Once synchro appears the amount of players using the extra deck will go up like 90%. You can easily predict what most players will summon by what monsters they summon, since it is a quick play card you'll disrupt their summon and glimpse at what their extra deck holds.