Immortal Gigas appears as a boss found on the 8th floor of Malebolge within the Abyss of Time. It is accompanied by three Visceral Mayas. After being defeated, it will drop a Steel Right Arm which can be sold.
Persona 4
Immortal Gigas appears in the 4th and 5th Worlds of Magatsu Mandala. It will boost its Critical Rate with Revolution before attacking with Physical skills. After being defeated, it may drop a Formidable Proof. Selling 4 Formidable Proof materials will unlock the Mikagura Vest male armor in Daidara's shop.
Persona 4 Golden
Immortal Gigas appears in the 4th and 5th Worlds of Magatsu Mandala. It attacks solely with both single- and multi-Physical skills, and has no method of increasing its Critical Rate.
A medium Bash attack. (1 Enemy) Recover HP equal to damage dealt.
Megaton Press
A heavy Bash attack, with medium chance of knockdown. (1 Enemy)
Bestial Roar
Act first and raise physical attack for 3 turns.
Trivia
Not counting enemies a boss can bring to the field via the Summon move, the 3 Mayas and the Immortal Gigas are the only bosses in Persona 3 or 4 to be in a group of four.