The mag is the important thing to have up on a pole. GPS reception probably doesn't get better just because it is on a pole, unless you have something like an FPV transmitter (or rarely your Rx or OpLink telemetry) that is causing GPS problems.
If you are worrying about 20g then you are probably flying a very small quad, and those are the ones that need the pole the most (for an aux mag) because without putting a mag on a pole you have to use the FC mag, and it is too close to the motor wiring, and that generates mag fields that can easily make it unflyable.
Most people who have experience will tell you that you need a mag on a pole for 330ish or smaller quads. I have an F330 with a long pole and it works very well. I haven't tried it without the pole.
I have a 500mm quad where I build all the motor wiring below the bottom plate and put a dome on top of the top plate for the FC and the GPS on top of the dome so GPS is about 6cm above top plate and about 11cm from motor wiring. No pole, but a lot of separation from motor wiring. Of course all motor wiring is twisted. I built one of these with a smaller dome for a friend, and it yaws a bit after takeoff. I would call that one marginal.