the doorsill
闑 refers specifically to a doorsill, the threshold piece at the bottom of a doorway.
闑 combines 門 (gate) with 臬, which likely contributes both sound and the idea of a threshold or standard. The exact historical development is uncertain.
The gate 門 has a wooden 木 target 目 (or a measuring post) inside: picture a wooden sill at the gate that you step over, the doorsill.
For ゲツ, imagine a gate with a geta (wooden clog) resting on the doorsill: geta -> ゲツ, and the geta sits right on the threshold.