Gutso is immobile and stays near the top of the arena for the entire fight while performing the following attacks:
Burps, letting out a spew of arcing brown projectiles and red bloody projectiles in random directions. The red projectiles linger high in the air a moment before dropping and creating a small puddle of red creep.
Spits out a Guts in Isaac's direction which lets out 4 diagonal bloody projectiles where it lands before traveling in a random direction until it hits a wall or obstacle and continues as usual.
If there are 3 Guts alive in the room, Gutso will hop in place before slamming down, creating a ring of 10 bloody projectiles and launching all Guts directly up into the air, which burst into 4 cardinal bloody projectiles upon landing.
Phase 2
Once his HP reaches 400 or below, Gutso's abdomen explodes, releasing a Mama Guts and revealing another "mouth" in the new orifice. Gutso will continue to use attacks from his first phase, with some variations:
When Gutso spews projectiles from his mouth, the arcing brown projectiles will instead come out of the second mouth in his stomach, reducing the travel time of all projectiles as the mouth is lower to the ground. The spew of brown projectiles will also be aimed in Isaac's direction, rather than being random.
The second mouth can spit out a Heart along with 2 projectiles that leave small puddles of red creep.
When Gutso hops, any Hearts in the room will be launched high into the air off-screen before landing into the mouth in his stomach, healing Gutso for 15 HP each and creating a spray of red projectiles in different directions as it consumes each one.
Notes
The Guts and Hearts while in the air are considered projectiles and can be destroyed by Lost Contact, preventing the enemy from spawning.