The pattern of gully distribution does not match the random pattern of meteor strikes.

They are situated in specific places, in bunches, regularly spaced along cliff lines and slopes.

It is possible that for ~1% of gullies that a meteor started the whole thing off a million years ago or so, but most just "grew" in place based on existing irregularities of the ground and patterns of snow accumulation. (CO2 or H2O, depending on your preferred model)

There is a big difference between the Mars of now with small gullies and rare impacts and the Noachian Mars with big impacts and many, but not ubiquitous, valley systems