They are not imaginary. There are threads like "RPUs and GPUs are totally useless" as well as threads such as "RPU success" or "GPU success" for the year. Your mileage may vary. Many of us are happy (myself) with them. Many of us are horribly upset (nomad420 and yourself).

It comes down to: flexibility. Are you booking on business heavy days where the plane is 100% sold out? Are you booking last minute? I've been upgraded to LHR both ways from LAX/SFO, so it does exist. It's not "imaginary"

A better way to search would be going to "Advanced" and then selecting "GPU" as an Upgrade. It will then give you green/yellow icons to indicate available GPU space or waitlisted GPU space respectively. Might be a bit faster than scrolling through each flight looking for R space.