How much can GaN components make powerbanks and chargers smaller and lighter?
It’s not the case that you make a new charger by filling some internal component full of GaN to replace the old silicon tech; there are probably only two components amongst maybe 30 or so that can be changed from silicon to GaN and these might weigh a few grams at most.

And, it’s likely that those components will remain inside the same physical package or, at best slightly smaller.

The biggest mass inside chargers is likely the flyback transformer and this dominates matters. It won’t change weight or dimensions with the arrival of GaN so sorry, I don’t think you are going to see anything other than the odd gram of weight loss.