Mad Gasser can be found in Tsukiji Hongwanji during the entrance test presented to Flynn and his fellow Samurai. Flynn will need the Demolingual App in order to communicate with them, but attempting to do so during the trial will result in them blowing on the candle. Mad Gasser can teach Flynn the Shibaboo and Poison Breath skills through its Demon Whisper.
Shin Megami Tensei IV Apocalypse
Mad Gasser can be found in Shibuya. He can teach the Bind Voice, Poison Breath and Pandemic Bomb through his Demon Whisper. Mad Gasser benefits greatly from learning ailment skills.
Devil Summoner: Soul Hackers
"A mysterious figure that spreads an unknown poisonous gas around. He wears all black and is said to be a tall man. The gas he uses has a sweet smell but inhaling it will cause intense headaches and vomiting. He returns when people begin to forget his existence, though he is rarely sighted."
—Devil Summoner: Soul Hackers compendium
Shin Megami Tensei: Liberation Dx2
"A mysterious tall man garbed in black that disperses an unknown poisonous gas. The inhalation of his sweetly scented concoction causes intense headaches and vomiting. He returns when people begin to forget his existence, though he is rarely seen."