How Do You Calculate Conditional Probability?

Let $R$ denote the event that I eat out in a restaurant on Saturday, and let $G$ denote the event that I go to the gym on Sunday. Suppose that the event of going to the gym after eating out is twice as likely as going to the gym without eating out (and that both of these events have positive probabilities). Also assume that $P(R) = \frac{2}{3}$. Find $P(G)$ in terms of $P(G \ | \ R^C)$.