If you have a PM, there's no need for a speed sensor for indoor training. You'll presumably train to power, and any software like Zwift, TR etc, is based on power.
You may still need one for outdoors, just to know how fast you're going!

This may be a dumb question, but how does a GPS headunit not give you accurate speed? I will keep the speed sensor then.

Thanks!

It does, you don't need a speed sensor.

Riding under trees and around many tall building makes GPS speed difficult.

If you regularly ride open roads with a clear view of the sky GPS speed will probably work very well for you.

It gives you accurate average speed, but instantaneous speed readings can jump around a bit if you have weak coverage when in built up areas, under tree cover or in tunnels for example.
However GPS based speed is generally fine. I prefer a sensor but I agree it's not essential if you have GPS.