Solubility of eight common gases in the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [emim][Tf2N], ionic liquid was systematically investigated based on alchemical free energy calculations from molecular dynamics simulations. The simulated solubilities and trend in terms of Henry's law constants agree qualitatively with the experiment. Polar gases such as H2S and nonpolar gases with a large quadrupole moment such as CO2 show the highest solubility, while nonpolar gases of small quadrupole moments (such as N2 and H2) are least soluble. The solute-ionic liquid interaction correlates with the observed solubility order. We also examined the temperature dependence of solubility for CO2 and N2 and found that the CO2 solvation in IL is exothermic with a negative solvation enthalpy, while the N2 solvation is endothermic, in agreement with the experiment.