cross-bar at end of poles of cart
軏 refers to a specific part of a cart: the cross-bar at the end of the poles. It is a narrow technical term with no broader conceptual center.
軏 combines 車 (cart) with 兀, which likely contributes the sound. The character is a specialized cart part term, but the exact historical development is uncertain.
The left side is a cart 車, and the right side 兀 looks like a simple cross-bar shape. Picture the cross-bar at the end of the cart poles.
For ゲツ, imagine a cart getting stuck because the cross-bar is loose: 'gets' stuck -> ゲツ, and the cross-bar needs fixing.