GaN is about size and heat, not extra speed

Gallium nitride is a semiconductor that switches faster and wastes less energy as heat than the silicon that came before it. In a charger, that means the transformer and heat sinking can be smaller for the same output. The result you notice is a 45W block the size of an old 20W one.

What GaN does not do is make a phone charge faster. The phone still negotiates for what it wants, and a 20W GaN charger and a 20W silicon charger hand over the same twenty watts. GaN shows up on a box because it explains the size, and it gets read as though it explains the speed.

Heat is worth understanding on its own, because it is the limit that governs real charging more often than any number on a box. A phone monitors its battery temperature and reduces its own charging rate when it gets warm. This is deliberate and protective. It is also why a phone in a hot car with a sunlit windscreen mount charges slowly no matter how large the number on the charger is.

A charger that runs cooler helps at the margins by not adding heat to a phone sitting next to it or on top of it. It does not override the phone's own thermal behavior, and no accessory can.

One boundary that is not negotiable: if a device or battery is hot to the touch, swollen, wet, or physically damaged, stop charging it. Do not charge under a pillow or blanket, where heat has nowhere to go. Do not open a battery, repair one, or replace cells. Those are not tasks for a kitchen table.