This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
忨 combines the heart radical 忄 with 元, but the exact historical development is uncertain. It may have originally expressed a greedy or desirous state of mind, but this is speculative.
The heart radical 忄 beside 元 (origin) might suggest a heart fixated on its original desires, but the meaning is unclear.
For ガン, imagine a greedy person named Gan who always wants more: Gan -> ガン, and his heart 忄 is never satisfied.