I'm no great petrophysicist but I think that if you were relying...

I'm no great petrophysicist but I think that if you were relying on the Archie equation for saturation (which you would in the case of clean rock with known parameters like regional water salinity), the presence of fresh water would affect your Archie saturation and you could miss a freshwater-bearing sand, particularly if it was in a variable sequence.

In theory yes, you could pick a GWC from density/neutron due to the gas effect. But if your logs are already showing a fair bit of variability in terms of den/neu, and if there's a reasonable degree of mud invasion, this would be hard to spot in the absence of a clear signal from the Archie equation.