jmc

To form a square with its vertices on the grid, we can start with a $1\times 1$, $2\times 2$, or $3\times 3$ square, then (optionally) cut off four congruent right triangles whose legs add up to the side length of the square we started with. These are all possible ways we can do it (up to congruence): The areas are $1$, $2$, $4$, $5$, and $9$. (In the case of the second and fourth squares, we can compute these areas by subtracting the areas of the right triangles from the area of the squares indicated by the dashed lines. Or, we can use the Pythagorean theorem to find the side length of each square, then square this to get the area.)

The sum of all possible areas is $1+2+4+5+9=\boxed{21}$.