Rapid degradation of magnesium implants can lead to premature mechanical failure and gas cavity formation, inducing inflammation in adjacent tissues. This study investigated corrosion and gas accumulation of Mg96Zn2Ga2 (wt.%) alloy with plasma electrolytic oxidation (PEO) or plasma electrolytic fluorination (PEF) coatings (thickness 7–8 µm). Coatings were characterized by XRD, SEM/EDS, potentiodynamic polarization, and electrochemical impedance spectroscopy. In vitro cytotoxicity and in vivo gas cavity formation were evaluated by implanting coated/uncoated magnesium screws into rat femurs, with weekly multi-slice computed tomography for four weeks. The PEO coating exhibited 14.6% porosity and was rich in MgO, while the PEF coating showed lower porosity (6.7%) and contained MgF2 and MgNaF3 phases. After 28 days of immersion in Hanks’ solution at 37 °C, corrosion rates were 0.015 mm/year for PEO and 0.010 mm/year for PEF. In vivo, gas cavity volume around uncoated screws increased progressively to over 2400 mm3 after 28 days, while PEO-coated screws showed a continuing upward trend and PEF-coated screws decreased to 184 mm3. In vitro direct-contact cytotoxicity revealed ∼60% proliferation inhibition for uncoated samples (due to gas bubbles) versus ∼30% for both coated samples. These results indicate that PEF coatings provide demonstrably improved long-term corrosion resistance and gas evolution control, termed Prevention of Gas Accumulation (PIGA), compared to PEO, thereby reducing hydrogen-related complications and enhancing clinical safety of biodegradable magnesium implants