Unlock extra missions by finding the gnome on the indicated level:
Aura Pool: Get the Gnome on level 8
Floor of Death: Get the Gnome on level 4
Bottomless Pit: Get the Gnome on level 4
Bouncy Bouncy: Get the Gnome on level 2
Gasoline: Get the Gnome on level 6
GearShift: Get the Gnome on level 3
Panic Room: Get the Gnome on level 3
Psi Pool: Get the Gnome on level 6
Stop Lights: Get the Gnome on level 7
Survival Mode: Get the Gnome on level 3
Tip The Idol: Get the Gnome on level 1
TK Alley: Get the Gnome on level 2
Up And Over: Get the Gnome on level 4
Gnomotron: Find all Gnomes in the game