Bimetallic catalysts provide new routes toward sustainable ammonia synthesis, but their nanoscale structural dynamics under reaction-relevant conditions remain poorly understood. Here, we combine in situ gas-cell and multimodal electron microscopy to determine how temperature, gas pressure, and chemistry select among distinct restructuring pathways in AuRu nanoparticle catalysts. Initially, the AuRu nanoparticles form polycrystalline face-centered cubic (FCC) alloys with Au/Ru intermixing. Elevated temperature (≥350 °C) induces intraparticle phase segregation into distinct Au-rich (FCC) and Ru-rich hexagonal close-packed (HCP) domains that exhibit localized plasmonic modes. Atmospheric-pressure 3:1 H2:N2 gas unlocks a distinct restructuring regime absent at lower pressures, characterized by pronounced faceting and nanovoid formation. Systematic gas variation identifies hydrogen as the dominant driver. Density functional theory-trained machine-learning interatomic potentials and Grand Canonical Monte Carlo simulations reveal that H–Ru interactions enhance the Au/Ru diffusivity mismatch, promoting vacancy accumulation and nanovoid formation. Together, these results show that, rather than simply accelerating the thermally driven phase segregation observed at lower pressures, atmospheric-pressure H2:N2 gas redirects restructuring toward faceting and nanovoid formation through a gas-mediated Kirkendall-type mechanism.