Beside the resistance headquarters' entrance there is a pub . Go left of it and you will reach a stone bridge. On the other side, by the water level, there is the Gnome .
One you shoot it down, turn right and you will see a crane by the river shore . Right below it there's a second sewer entrance. Once underground, go left on the first intersection  and forward on the second.
The Gnome is hanging below the ceiling, where a small ray of light is falling into the sewers  .
Return to the pub by the headquarters and turn right after leaving it. Go forward all the time and you will reach a factory with an open door and a stone bridge beside . Go to the other side of the canal. On the right wall of the building in front of you hangs the next Gnome ready to be shot down (above the skull poster) .
Head to Nigel's band hideout which you have visited during "Kidnapped"  and jump down through the hole .
Turn right at the first intersection and go straight through the next one . At the end of the path you will find the Gnome .
The last Gnome is in one of the cages in the cake factory . To get there, complete "Animal Liberation". The Gnome is in the corner in the room with the lever .