Flooding the Gates is a rare spell that deals damage to all units, enemy or friendly, bordering your base. The damage done can be increased by leveling it up. Flooding the Gates can fit well into any kind of deck, but usually control.
Flooding the Gates is useful for dealing with enemy units that have pushed all the way up to your base.
Flooding the Gates will also deal damage to friendly units as well, so you should avoid having units there when playing it.
When your whole home row is filled with enemy units, Flooding the Gates is great for opening it up and pushing your enemy back.
You can try engaging in a "base race" with your opponent, going 100% aggressive with them. Once both bases are filled with each other's units, you can play Flooding the Gates for huge value.
The limited opportunities to play this card efficiently and the fact that it inflicts friendly fire on your own units make this a very limited card.