To rescue Garth you need to visit the Jungle Holding Site sub-locale. There's a couple of small puzzles to pass by involving security lights and laser beams.
Thank you for the answer!
Unluckily, I've no idea how to pass that either. I know that you can destroy the thing that powers the lights and makes them flicker, but everytime I try to run past them I get caught by them. Even if I'm seemingly out of range I get blocked by the rock on the upper side of the screen and can't progress forward.