This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
岏 combines 山 (mountain) with 元, which may contribute a phonetic element. The exact historical development is uncertain.
The left side is a mountain 山, and the right side is 元 (origin). Picture a mountain that is the origin of a steep, high peak: 岏 suggests a towering, precipitous mountain.
For ガン, imagine a steep mountain that looks like a giant gun barrel pointing up: gun -> ガン, and the mountain is high and steep.