Most gullies in the southern mid-latitudes are on south-facing slopes, which are the coldest and have the most frost in the winter. However, some occur on other slopes.
This image shows large gullies on both the pole- and equator-facing slopes. An important puzzle in Mars science is whether or not all of these gullies form in the same geologic eras and by the same processes.
If you have red/green glasses, be sure to check out the anaglyph of this crater, which shows rugged topography! [More at link]