From the Super Mario Wiki
Gobblegut is a boss from Super Mario Galaxy 2. It is fought in the Battle Belt Galaxy during the secret mission Snacktime for Gobblegut. It is a fiery version of the original Gobblegut found in Bowser Jr.'s Fiery Flotilla and has the same name as it in both English and Japanese. Gobblegut is one of two bosses in Super Mario Galaxy 2 fought in a secret mission, the other being Bugaboom.
Gobblegut has several changes to its body from its normal form. First, its skin color is now lava red instead of spring green. Also, its body now radiates lava, burning Mario at the touch and creating pools of lava when it bites the planet. It is only hurt when its bulges are spun, though the bulges are protected by surrounding fire.
This version of Gobblegut is fought in the same manner as its non-fiery counterpart. Six bellyache bulges are found at various intervals along Gobblegut's serpentine body. When Gobblegut burrows into the ground, the bellyache bulges will become stuck in the ground, at which point the player must attempt to spin each of the bellyache bulges, popping them and causing them to disappear. Mario can also perform a backflip and then spin to hit the bellyache bulges when they are above him. Each time one is destroyed, 10 Star Bits are released, and Gobblegut takes more damage. When three or fewer bellyache bulges remain, Gobblegut begins to make fireballs rain, creating pools of lava where they land. When all six of Gobblegut's bellyache bulges are destroyed, it belches and explodes, and Mario is granted a Power Star.
Names in other languages