Back when Anker came out with a GaN based USB charger I bought one and took it apart, I was actually pretty underwhelming. It used a highly integrated power IC that has all the power FETs, control circuity, and isolators. I suspect that most modern chargers are something similar.
The one I took apart used a chip from Power Integrations, the primary side switch is GaN and it runs as a quasi-resonant flyback converter
https://ac-dc.power.com/products/innoswitch-family/ . I think some might also use an active clamped flyback design, that topology is popular now.
Given the high level of integration they are pretty simple inside and I suspect that the GaN version of the power IC only costs a bit more than the Si version.