What do you mean by "gain" in the speed control example? What relation to sensing does it have?

In this example, we let the engine force, $F_{\mathrm {eng} }$, be proportional to $(v_{\mathrm {des} }-v)$. The "gain" is the proportionality constant $k_{p}$. The use probably follows from that in electronic amplifiers.

We do not consider sensing problems in this example. Instead we assume perfect sensing/measurement of the output, which is the vehicle's velocity, $v$. The gain is only related to the control law.

--Shuo