Gornog's third ability doesn't have a duration. It will remain in effect until the game ends, the target creature leaves the battlefield, or some subsequent effect changes the creature's characteristics, whichever comes first.

Gornog's third ability overwrites the target's existing creature types. Once the ability resolves, that creature will be a Coward and won't have its previous creature types. It retains any other types and supertypes it has as well as any subtypes that aren't creature types.

The bonus granted by Gornog's last ability changes as the number of Cowards your opponents control changes.

Once a creature has blocked a Warrior, turning that creature into a Coward won't remove it from combat or cause the Warrior to become unblocked. (The Coward might suddenly have some regrets, though.)