GUFs have always booked into Z (which is Avios availability bucket).
The exception is booking via a TA, where they can use the lowest revenue bucket.
At this point, the lowest revenue bucket on those flights is F - as A no longer exists. This would increase GUF value as they could pretty much on any seat in the F cabin.
In reality until we get feedback from TAs trying to do this in the real world we won't know for sure.