Transconductance of devices
No, you don't always need to know the gm of a transistor.

Transconductance or gm is a simplification, or more precisely it assumes linear behavior of a device. So for a device that is switching transconductance isn't of much use as switching isn't linear behavior.

When for example using a MOSFET for switching, what is important for the speed is the charging and discharging of the gate. Which depends on the source you're driving the gate with, not the gm of the MOSFET.

For an amplifier stage it is important to estimate the gm of each device in the signal path as the transistors in an amplifier are supposed to work as linear devices.

A degeneration resistor (in series with source or emitter) does not eliminate the need to know gm. What if the degeneration resistor is much smaller than 1/gm, then the total resulting gm is still mainly determined by the transistor's gm and not the degeneration resistor.

You have to understand when you can ignore the gm of a transistor and when not. As a beginner you have to try and take every gm (in the signal path) into account. With experience you'll learn to recognize which transconductances you can ignore and which you cannot.