Yes, the BIOS can be flashed without a CPU. It is described by Gigabyte as Q-Flash Plus.
Pay attention to which revision of the mainboard you have (Rev 1.1 has different BIOS versions than Rev 1.0) but you can't flash anything (I also used the wrong BIOS at first).

But it also works if you install CPU/RAM and connect a monitor. The PC starts and you can enter the BIOS or the BIOS Flash Menu and perform the BIOS update from there.

I received my Z790 AORUS ELITE AX last week. It is important that you check which rev. 1.x you have. It is written in the bottom left corner where the audio connector is located. In my case it is rev. 1.2. According to Gigabyte, the I9-14900 is already supported. So you don't have to update the BIOS first.
I have the I9-14900KF but haven't been able to try it out yet as I'm still waiting for the monitor which will hopefully be delivered next week.

On the Aorus homepage you could already find it (https://www.aorus.com/motherboar...), supports 14/13/12 Gen processors...