Martian slope gullies are argued to be evidence for recent liquid water flow on the surface of Mars. To explain the source of water, a wide range of environmental conditions and processes has been invoked. However, a lack of information about the environmental context or timing of gully activity makes it difficult to evaluate the theories. Here, we present new observations of extensive gully modification over the past 6 Mars years within dune gullies with slope-gully morphology. Observed activity within 18 gullies in 7 dune fields constrains timing to winter, which is consistent with observed slope-gully activity. These observations show that fluvial processes are unlikely to cause present-day Martian dune-gully activity, and imply that CO2 frost accumulation may play the dominant role.