This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 卾 is uncertain. It visually combines 咢 and 卩, but the original meaning and component function are unclear.
The left side 咢 suggests a jaw or chin, and the right side 卩 resembles a kneeling person. Together they might hint at the jaw area, but no clear meaning is established.
For ガク, imagine a jaw (がく) dropping open: the sound ガク is the same as the Japanese word for jaw, which fits the visual hint of 咢.