The Victory Gong is an audio or visual cue designed to subtly (or unsubtly) signal to the player that a combat phase has ended, and that regular exploration/interaction can begin. As soon as the Victory Gong is sounded, allies and party members who have been killed or knocked out will often revive, as if the battle never happened. Oftentimes they will also regenerate health and mana.
Victory Gongs are a relatively recent invention, only coming into fashion as real-time combat became the norm, and as shooters and action titles became more capable at delivering narrative. Prior to that, turn-based games did not expressly require a Victory Gong, since the division between combat and regular gameplay was often quite obvious. A Victory Gong is different from victory music as the Victory Gong is simply too short, or not musical at all.
Though Victory Gongs were present in one form or another in many earlier games, their use became apparent with the advent of Gears of War, a cover-based third-person shooter that signaled the end of combat with an almost literal "GONG" sound. Other games that mimic Gears of War, including Terminator: Salvation and Hunted: the Demon's Forge, have often been seen implementing Victory Gongs.
Role-playing games often use their musical scores as a form of Victory Gong, adding in a musical "stinger" to accompany the character's sheathing weapons or standing up straight.