The Dragon Gong is a magical item that belongs on Mystery Mountain, which also contains the place's symbol. It was held in the museum since the Season 2 episode "The Dragon Gong." However, in the Season 3 episode "Meet An Yu," Night Ninja brought the gong to Mystery Mountain in order to unleash its magic, and has remained there ever since. It is also where An Yu had been trapped for centuries, but because Night Ninja returned the gong to the mountain, she was then freed. Since An Yu is still the same age, even centuries before now, it is possible that the Dragon Gong prevented her from aging.
In "The Dragon Gong," Night Ninja stole the gong from the museum to summon the dragon. It was later returned there after the PJ Masks defeated Night Ninja and his Ninjalinos by ordering the dragon to blow them away.
In "Meet An Yu," Night Ninja and his Ninjalinos once again stole the ancient gong and brought it to Mystery Mountain in order to unlock the secrets of the mountain. After the dragon was summoned and ordered to blow the PJ Masks away, the Dragon Girl was also freed.
In "Clash on Mystery Mountain," An Yu was again trapped in the gong when Night Ninja hit it with the mallet. After a brief confrontation between the villain and Teeny Weeny that ended with the latter chanting a spell to transport them out of the mountain, the heroes retrieved the gong's mallet, and An Yu was again set free.