Determine transconductance of a JFET given only voltage gain and internal drain resistance
I think I figured it out.

Since we are talking about a JFET in isolation, Av=VDS/VGS

Since ID=gm*VGS, we can rearrange to
VGS=ID/gm
and since VGS is always negative, we can make this VGS=-ID/gm

lastly, since VDS=ID*rd we can plug these 2 equations into the first one to obtain
Av=ID*rd/-ID/gm

rearranging gives Av=-rd*gm which we can plug in -200 and 100kΩ to get a gm=2mS.