  By developing a gain-embedded cavity magnonics platform, we create
gain-driven polariton (GDP) that is activated by an amplified electromagnetic
field. Distinct effects of gain-driven light-matter interaction, such as
polariton auto-oscillations, polariton phase singularity, self-selection of a
polariton bright mode, and gain-induced magnon-photon synchronization, are
theoretically studied and experimentally manifested. Utilizing the
gain-sustained photon coherence of the GDP, we demonstrate polariton-based
coherent microwave amplication (~ 40 dB) and achieve high-quality coherent
microwave emission (Q > 10^9).
