On rare occasions an Other World encounter can cause the Nightgaunt to appear. Failing a Combat check to the Nightgaunt in an Other World will cause the investigator to return to the gate in Arkham with an explored marker. The closest gate referred to by the card would be the gate back to Arkham in this case.
Also, according to Kevin's answers, if an investigator fails a combat check against a Nightgaunt while in Arkham and no gates are open, or while in an Other World and no gates are open, the combat immediately ends with no effect. (Note that this can occur if all gates have been closed but the investigators do not collectively have enough gate trophies to win.)
Nightgaunts first appear in the story The Dream-Quest of Unknown Kadath (1927), written by H.P. Lovecraft.