Guck is found in the Swamp in glowing green clumps attached to large trees. To gather guck you will need to climb the tree because you cannot cut down guck-covered trees. Be careful of low-hanging guck because there are usually leeches in the water with deadly poison.
Guck is needed for all green aesthetic structures but most importantly for the Draugr Fang Bow.
You will need to build a workbench near the base of the tree which might also require leveling off the ground with a hoe. After that, there’s not really a strategy for building scaffolding to climb guck-covered trees, but here’s a look at how I used a bunch of ladders to reach every deposit: