Overview plot of the maximum gap depth in the ideal velocity-channel maps when comparing all transitions considered in this work. The central radiation source is a T Tauri (left) or a Herbig Ae star (right). For the most massive disks in our sample. All considered transitions are optically thick and only trace the surface of the disks. Thus, for the smallest disks the gap is detectable outside the line in the dust continuum (marked with Dust).