Step 1: Determine the fraction of substance G in the solution. Given that substance G makes up $\frac{2}{3}$ of the solution. Step 2: Determine the fraction of substance F in the solution. The rest of the solution is substance F. So, $$ \text{Fraction of F} = 1 - \text{Fraction of G} $$ $$ \text{Fraction of F} = 1 - \frac{2}{3} $$ $$ \text{Fraction of F} = \frac{3}{3} - \frac{2}{3} $$ $$ \text{Fraction of F} = \frac{1}{3} $$ Step 3: Express substance G as a fraction of substance F. This means we need to find the ratio $\frac{\text{Fraction of G}}{\text{Fraction of F}}$. $$ \frac{\text{Fraction of G}}{\text{Fraction of F}} = \frac{\frac{2}{3}}{\frac{1}{3}} $$ To simplify, multiply the numerator by the reciprocal of the denominator: $$ \frac{\frac{2}{3}}{\frac{1}{3}} = \frac{2}{3} \times \frac{3}{1} $$ $$ \frac{\frac{2}{3}}{\frac{1}{3}} = \frac{2 \times 3}{3 \times 1} $$ $$ \frac{\frac{2}{3}}{\frac{1}{3}} = \frac{6}{3} $$ $$ \frac{\frac{2}{3}}{\frac{1}{3}} = 2 $$ So, substance G is 2 times substance F. As a fraction, this can be written as $\frac{2}{1}$. The final answer is $\boxed{\frac{2}{1}}$