Why is "GaN tech" so revolutionary for wall chargers?
GaN is used in the FET switches to replace silicon.  GaN has higher switching speeds, meaning that the switching frequency can be higher leading to smaller magnetics. It has low output capacitance which combined with the faster switching means lower switching losses.  The output capacitance is much more linear with voltage than Si as well.

It also has very good RDSon for a high voltage FET meaning conduction losses are low.

So combine smaller magnetics with lower losses and you can fit an adapter into a smaller package without the thermal problems you would have with silicon FETs.  Or you can keep the size the same and opt for better efficiency.

It's not a miracle, but it is incrementally better.  It also does have marketing appeal: "Uses the latest gallium nitride technology!"