If Koffing is your Active Pokémon and is Knocked Out by an opponent's attack, the Attacking Pokémon Is now Confused and Poisoned.
Gnaw かじる
10
Trivia
The Knockout Gas Poké-BODY was later errata'd so that only a Knock Out by damage would trigger the power; prior to the errata, any Knock Out would trigger the power, including placing damage counters and specific effects that stated a Pokémon was Knocked Out.