I am skeptical about assertions that these gullies, as well as the dark slope streaks have anything to do whatsoever with briney sub-surface water sources. They always seem to originate at the top of a slope, or a crater rim, not the most likely location for a subsurface aquifer. The work that's been done raises serious questions in that these features appear to occur most frequently in areas where we have not detected subsurface H2O.

But, I think the jury is out on this one and it may remain out for some time.

I have been most intrigued by the following idea:

Snow Gullies on Mars

But, again, more data is needed.