As far as etymology it is a man with the head emphasized. There are several other "coincidences":

1) As a farmer tilling the land: 土 + 人
2) As a sedan carrier shouldering two poles: 二 + 人
3) As husband and wife being two persons: 二 + 人
4) As a man communicating with the heavens: 天 + 人

Choose whichever you like, but in "gongfu" from a classical pov, it appears to simply be a noun suffix, rather than adding to the literal or implied meaning of the word, as it is not a person like "nongfu" (farmer).