In flowing medium Chemical Oxygen Iodine Laser (COIL), Singlet oxygen is produced by the exothermic reaction of basic hydrogen peroxide solution and chlorine gas. It pumps the iodine and lasing process takes place by stimulated emission. Laser power is extracted using cavity. Development of customized data acquisition system is essential for measurements and analysis of both fundamental (temperature, pressure, level) as well as derived parameters (lasing medium concentration, flow rates of gases and laser power). The focus of the present paper is to dwell on uncertainty evaluation of a complex gas laser source in terms of ascertaining influences of primary/fundamental variables and corresponding derived parameters along with manner of uncertainty propagation. The study facilitates determining the variables with most significant impact on system performance, critical form point of view from optimal functioning of large-scale systems. This enables prediction of overall system uncertainty potentially extendable to other similar laser systems involving subsystems with mutual interdependencies together being distributed over a significantly large laboratory space. The relative combined uncertainty is computed to be 8.3%. The methodology shows significant potential for true decision-making and control of realistic gas laser source operation using developed 150 channel Data Acquisition and Analysis System (DAAS).