Temperatures and pressures are within the range to permit liquid water in Gusev, Mars.
This is from data known since the time of Viking.
You only need the temperature to be above freezing *some* of the time to have liquid water. Pathfinder and Viking both showed that at elevations such as Gusev, the pressure is always above 6.1 mbars.
It is only the belief that liquid water can not exist on Mars that prevents acceptance of this fact.