This article presents a performance study of using different working fluids (gases) to power on Concentrated Solar Gas Engine (CSGE-Stirling and/or Brayton). Different working gases such as Monatomic (five types), Diatomic (three types) and Polyatomic (four types) are used in this investigation. The survey purported to increase the solar gas engine efficiency hence; decreasing the price of the output power. The effect of using different working gases is noticed on the engine volume, dish area, total plant area, efficiency, compression and pressure ratios thence; the Total Plant Cost (TPC, $). The results reveal that the top cycle temperature effect is reflected on the cycle by increasing the total plant efficiency (2-10%) for Brayton operational case and 5-25% for Stirling operational case. Moreover; Brayton engine resulted higher design limits against the Stirling related to total plant area, m(2) and TPC, $ while generating 1-100 MWe as an economic case study plant. C2H2 achieved remarkable results however, CO2 is considered for both cycles operation putting in consideration the gas flammability and safety issues.