A neutronic feasibility study was performed for a 600-MW(thermal) gas-cooled fast reactor fuel cycle through recycling simulations. Sensitivity calculations were also performed for various physics design parameters such as the plutonium volume fraction of the fuel, fuel burnup, core material volume fraction, and the power density. The results showed that the initial breeding gain of -0.04755 is sufficient to sustain the recycling of the actinides with a reasonable amount of natural uranium and plutonium feed material. The comparative calculation on the core power density has shown that it is feasible to reduce the amount of minor actinides and spent fuel in the high power density core (98.4 MW/m3) compared to the reference core (58.2 MW/m3). It was also found that the fuel cycle cost is saved by 0.4 mills/kWh for the high power density core compared to the reference core.