The gain is calculated by solving the Schichmann-Hodges equations for the gain given the Rds at a particular Vgs and Id. Specifying all of these points removes any ambiguity about where the Rds is defined (i.e., is it the slope at small Vds or is it the slope somewhere higher up the characteristic). In our case, it is the slope at a particular point (which is also why we need more data to specify exactly where the Rds is defined...it is not just a constant).
Note, too, that the intrinsic Rds for this calculation (i.e., for just the channel) must have the series source and drain resistances removed from the gain computation as well. This is all done internally to the block.
