The Sample Analysis at Mars (SAM) instrument onboard the Mars Science Laboratory Curiosity rover measures the chemical composition of major atmospheric species (CO₂, N₂, ⁴⁰Ar, O₂, and CO) through a dedicated atmospheric inlet. We report here measurements of volume mixing ratios in Gale Crater using the SAM quadrupole mass spectrometer, obtained over a period of nearly 5 years (3 Mars years) from landing. The observation period spans the northern summer of MY 31 and solar longitude (L_S) of 175° through spring of MY 34, L_S = 12°. This work expands upon prior reports of the mixing ratios measured by SAM QMS in the first 105 sols of the mission. The SAM QMS atmospheric measurements were taken periodically, with a cumulative coverage of four or five experiments per season on Mars. Major observations include the seasonal cycle of CO₂, N₂, and Ar, which lags approximately 20–40° of L_S behind the pressure cycle driven by CO₂ condensation and sublimation from the winter poles. This seasonal cycle indicates that transport occurs on faster timescales than mixing. The mixing ratio of O₂ shows significant seasonal and interannual variability, suggesting an unknown atmospheric or surface process at work. The O₂ measurements are compared to several parameters, including dust optical depth and trace CH₄ measurements by Curiosity. We derive annual mean volume mixing ratios for the atmosphere in Gale Crater: CO₂ = 0.951 (±0.003), N₂ = 0.0259 (±0.0006), ⁴⁰Ar = 0.0194 (±0.0004), O₂ = 1.61 (±0.09) x 10^‐3, and CO = 5.8 (±0.8) x 10^‐4.