Energy efficiency is a fundamental objective for 6G systems and requires adaptive radio operation beyond conventional modulation and coding. While existing approaches rely on duty cycling or component deactivation, substantial savings require hardware-aware adaptability, particularly under low and moderate traffic conditions. This paper investigates a practical Gearbox PHY architecture with a finite set of hardwareaware operating modes ('gears'), each defined by distinct power amplifier (PA) configurations and corresponding achievable SINR ranges. Unlike theoretical designs that assume unconstrained hardware reconfiguration, the proposed approach is compatible with 3GPP NR resource structures and standard signaling. Energy-efficient operation is formulated as a joint optimization of gear selection and discrete time-frequency resource allocation. A real-time testbed based on an OpenAirInterface gNB with dual switchable PAs and synchronized power measurements validates the concept and confirms that multi-gear operation achieves significant energy savings compared to conventional duty cycling using a single transceiver optimized for peak performance.