Formally, a gas is a fluid (one that is not a liquid), but the use of the suffix '$f' in variable and expression names when dealing with water-related Psychrometrics quantities typically implies that one is dealing with liquid (fluid) water.
The use of 'g' and 'f' subscripts in formulae is common in many references, but some use 'v' for water vapour and 'l' for liquid water, which is more consistent (because there is no confusion concerning "fluid") but it is slightly less commonly used in Psychrometrics.
Note also that:
The distinction between 'vapour' and "just gas" is: