This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
遌 combines the movement radical 辶 with 咢, which likely contributes a phonetic element. The exact historical development is uncertain.
The movement radical 辶 surrounds 咢, but without a clear meaning, picture a mysterious figure moving along a path, its purpose unknown.
For ガク, imagine a person walking along a path and suddenly exclaiming 'Gak!' in surprise when they encounter something unexpected: Gak -> ガク.