Digimon World Dusk: Machine Quest: Find the Iron Pillow.

Now, go talk to Andromon and start the quest. Then, go find Kapurimon in Dark Square. He will ask you to go to Sunken Tunnel and get his Iron Pillow.

When you reach there, climb down the ladder and take the top southwest to the next screen. Follow the path to southeast. Then, you will find Gizamon in the area, climb that ladder and talk to him and he will fight you.

Gizamon
HP: 1000~
EXP: 180 Aquan
Bit: 70 bits.

Now, go back to Kapurimon to give his pillow and go to Dark Center to complete the quest with 95 Tamer Points, 3685 bits and a Novice Cloak.

*Now you be able to find Gizamon in Sunken Tunnel!