This study presents the detailed characterization of a magnetron-based Ar–N2 plasma discharge used to sputter a liquid Ga target for the deposition of gallium nitride (GaN) thin films. By utilizing in situ diagnostic techniques including optical emission spectroscopy and microwave interferometry, we determine different temperatures (rotational and vibrational of N2 molecules, and electronic excitation of Ar atoms) and electron density, respectively. Beyond providing insights into fundamental plasma physics, our research establishes a significant correlation between gas-phase dynamics, particularly those of gallium atoms (flux and average energy at the substrate) and deposited GaN thin film properties (growth rate and crystalline fraction). These findings underscore the role of plasma conditions in enhancing thin film quality, highlighting the importance of plasma characterization in understanding and optimizing GaN thin film growth processes.