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

From MurrayWiki

In this example, we let the engine force, F_eng, be proportional to (v_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