After completing the main quest “Defeat the Underpants Gnome”, talk to gnome outside the mouse hole in your room to receive this quest. He'll ask you to collect five underpants. They can be found on the following locations:
1: Mrs. Cartman's Room
2: Kevin's Room
3: Stan's Room
4: Stan's parents' room
5: Kyle's Room
Reward
- New Friend: Underpants Gnome