In this paper, the output tracking control problem for single-input single-output (SISO) systems with unknown high-frequency gain is investigated. For the situation where the system matrix has unknown parameters and the high-frequency gain cannot be measured, based on the output feedback control strategy, a novel reduced-order model reference adaptive control (MRAC) approach that relies solely on the sign of the gain is proposed. This approach only requires updating a scalar function, which substantially alleviates the computational burden of the designed controller. Furthermore, the control strategy ensures closed-loop stabilization of the system and enables asymptotic tracking of the system's output state. The effectiveness of the proposed method is finally validated through two simulation examples.