How to reach Lower Guk
The fastest way to Lower Guk is first by traveling to PoK and then taking the Grobb stone (in southeastern PoK on the evil side) which dumps you out into Innothule Swamp. Once you're here travel northeast to Upper Guk (the blue circle on my map above).
Once you're in Upper Guk getting to Lower Guk is a bit of a pain in the butt. Use my map below for better directions than what I am going to tell you here - but basically you need to go down a floor in Upper Guk in order to reach the Lower Guk zone line.