I've just had a discussion with the staff at the GF desk in T5. They explained* the rules as follows:

GGL can guest into GF up to 5 as long as on same PNR.
GCH can guest up to 5 into GC as long as on same PNR.
Applies until 13th Jan.

All dependent on capacity.

They did seem very confused on the new rules and couldn't quite work out if I as a GCH could actually bring my family (3 bronze) into GF but told me it was too busy today anyway. Since we're already in CW the ability to guest into GC didn't change the status quo for us.

* with much discussion, consultation of their crib sheet and an admission that as the rules were new, they weren't really sure about them yet.