Need help finding the transconductance author of this book is referring to
\$g_m\$ is a small-signal parameter that will depend on transistor biasing.  Additionally, it will vary from device to device.  Circuits must be designed to not depend on \$g_m\$ having a precise value, same as bipolar circuits must not depend heavily on \$\beta\$ as this will vary with biasing and temperature.  \$g_{fs}\$ is the manufacturer measuring \$g_m\$ at one specific bias condition, to give a ballpark value of \$g_m\$.  \$g_{oss}\$ is related to the output impedance, where \$g_{oss} = 1/r_d\$, again at one specific bias condition.