You can use any GaN power brick as long as you use a USB-A to USB-C cable. The router does not have true USB-C, its basically out of spec 5V (aka doesn’t have a PD chip, nor the resistors to tell USB-C to output 5V that are required). If your adapter only has USB-C ports, you could use an OTG adapter + the cable to get it to work.
The reason the GL routers are not using true USB-C with PD for example is every time you plug something into another USB-C port, the router would reboot. If this happens while you are changing configuration on the router, files will be corrupt and you would need to unbrick the device.
Also, the router only really needs 2A. The extra 2A are for external harddisks and other things you might connect to USB. Users in the past have had to use powered usb hubs to use external disks, which was a pain.