This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
枙 combines 木 (tree/wood) with 厄, but the exact historical development and original meaning are uncertain.
A tree 木 beside 厄, a shape associated with hardship. Without a clear meaning, picture a gnarled tree that has endured hardship.
For ガ, imagine a gnarled tree that makes a low 'ga' sound as it bends in the wind: ga -> ガ.