Solution:

We know that $\gamma - 1 = \frac{R}{C_v}$. Given $\frac{R}{C_v} = 0.67 \approx \frac{2}{3}$, we get $\gamma = 1 + \frac{2}{3} = \frac{5}{3}$. A heat capacity ratio of $\frac{5}{3}$ corresponds to a monoatomic gas.