This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
誐 combines 言 (speech) with 我, which likely contributes the sound. The exact historical development is uncertain.
The left side is 言 (speech), and the right side is 我 (self). Picture someone speaking about themselves, but the exact meaning is unclear.
For ガ, imagine a person speaking (言) about their own ego (我), and the sound they make is a drawn-out 'gaaa': ガ.