Mountain radical on top, the indexing radical. A 岗 is a small hill or mound — a baby mountain — so the radical does literal semantic work. The 'sentry post' sense grew from soldiers being stationed on hilltop lookouts, and the 'job position' sense from there.
Lower 冈 supplies the gāng sound (which becomes gǎng with the descending tone in 岗). It also already means 'hill ridge' on its own — making this a doubled semantic-phonetic compound. Adding 山 above emphasized the mountain aspect when 冈 alone felt too abstract.
