Hop over the fence in the hallway just before the turret/elevator and TK the gnome to you (it can be seen through a floor vent).
Tip the Idol: The gnome in Level 1
TK surf into the second story of the cryo lab once the countdown begins. The gnome is behind the wall where Leonov stood.
TK Alley: First gnome in Level 2
It's in a niche between the two static filled jumbotrons at the very end of the level. Walk up the broken floor that forms a slant and TK the gnome.
Bouncy, Bouncy: Second gnome in Level 2
It's on the roof of the shipyard entrance near the guard post where you obtain MC. Stack up boxes to reach the rooftop, turn around, and TK the gnome.
Gearshift: First gnome in Level 3
Inside the shipyard there's a hidden area enclosed by the large shipping crates. Face the office, it's to the left of guard tower with the crane lift.
Panic Room: Second gnome in Level 3
After destroying the four "chompers" in the room with the dock worker cutscene, the gnome is inside a secret passage connecting the pair on the right.
Bottomless Pit: First gnome in level 4
In the conveyor belt room, there is a pair of ladders in the far right corner. Hop off the conveyor belt to scale the fence. The ladder leads to it.
Up & Over: Second gnome in level 4
Destroy a vent at the marble atrium in the hall leading to the illusion chamber control room. Enter the shaft and turn right twice to find the gnome.
Floor of Death: First gnome in level 5
Start walking forward when the graveyard battle begins. You'll see the gnome behind a tree in the far corner (near a medical kit).
Survival: Second gnome in level 5
After the fusion reactor is deactivated, walk through the door and climb down the ladder to find the gnome in the pool.
Gasoline: First gnome in level 6
Destroy the triple turrets to the right of the entrance to Marlena's chamber. The gnome is in the back left corner of the adjacent computer room.
Psi Pool: Second gnome in level 6
Kill the snipers at the spawn then make a staircase of boxes. The gnome is on the facade's third story left (search with the sniper scope then TK it).
Stoplights: The gnome in level 7
It's behind a boulder just before you enter the alien gateway (long green & black catwalk before the elevator to the final battle).
Aura Pool: The gnome in level 8
Collect all thirteen main mission gnomes (and save your game each time).
Gnomotron