stubble, stump of a tree (hence: sterility), square stool
杌 centers on a cut-off remnant: a tree stump, stubble, or something barren. The square stool sense likely comes from a stump used as a seat.
杌 combines 木 (tree) with 兀, which likely contributes the sound and the idea of something cut short or truncated. The exact historical development is uncertain.
A tree 木 beside a truncated shape 兀: picture a tree cut down to a stump, leaving only stubble and a barren remnant that can serve as a square stool.
For ゴツ, imagine a gnarled old stump that looks like a goblin's seat: goblin stump -> ゴツ.