An investigation of GaN-HEMT based switching stages in hybrid switching amplifiers (HSA) for wide-bandwidth supply modulation of RF power amplifiers is presented. The HSA is designed for maximum 15 V supply voltage, 1 A supply current and 10 MHz bandwidth. GaNHEMT switch designs with alternative gate drive circuitry are investigated with regard to power, efficiency and linearity under different load conditions using simulations and measurements of the fabricated circuits. The full HSA showed an efficiency of 49.7% for a WCDMA envelope. A modified switching stage with improved gate-drive circuitry shows 85% efficiency in a 15 V buck-converter test-circuit for a 10 V conversion at 1 MHz over a 15-load.