Left wood radical - pictograph of a tree with branches above and roots below. The indexing radical: a 杠 is originally the wooden crossbar at the foot of a bed, then any thick pole or bar. Same radical: 板 plank, 杆 pole, 棍 stick.
Right 工 supplies the sound - gōng drifts to gàng with tone change, a common pattern (cf. 江, 红, 项). 工 itself depicts a carpenter's square, giving a faint hint of crafted lumber alongside its main job as the reading.
