JFET resistance and transconductance
Unfortunately, the gm equation you used is only valid for gm, and over values of Vds and Vgs that are appropriate when the FET is being used in its linear amplifier mode, that is, when it's useful to know gm.
If you want to know Rds, especially for large negative Vgs which makes for a high Rds, then you'll need a different equation.