why arcane legacy steps to boot from USB with GPT and UEFI?

To run the live AVG rescue "disk" on USB for Windows 10, how is it made to work with a GPT and UEFI system? (Possible solution listed below.)

I enabled the boot menu so that pressing F12 will display a list to select from. Also, I put everything USB at the top of the boot sequence and Windows and the HDD at the bottom. However, I hadn't realized GPT and UEFI are so different from a BIOS.

Because the USB flash drive isn't compatible with GPT it's not being picked up?

This solution was suggested:

Open the notebook and press quickly F2 to enter the BIOS.
Under the MAIN menu, enable the F12 Boot Menu option.
Under the BOOT menu, select Legacy boot and press F10 Save & Exit.
While the notebook is rebooting, press F2 again to enter the BIOS.
Under the BOOT menu, select as your first boot option the USB HDD stick (your USB stick).
Press F10 Save & Exit.

Perhaps this will work, but what is the problem that prevents UEFI boot from registering the thumb drive? Because it's using GPT and not an MBR?

I don't want to switch to legacy, but stay with UEFI. The laptop:

https://www.acer.com/ac/en/CA/content/model/NX.MLFAA.003

Yes, it is likely that the "rescue" USB stick only has a BIOS boot sector but not an UEFI bootloader. Many custom live environment makers either don't know about UEFI, or don't care about it yet.

So if your firmware is in "UEFI only" mode, then you will need to switch to a different mode in order to boot a USB stick that's not UEFI-compatible. (Yes, it might be possible to make the USB stick itself compatible with UEFI boot, but that's going to take much longer and is specific to the particular image you're trying to boot.)

Some UEFI firmwares only have "UEFI-only" and "BIOS-only" modes. Others always work in "dual" mode, that is, they automatically detect both UEFI-compatible and BIOS-compatible disks in the boot menu. Yet others have all three. It varies.

Try disabling "Secure Boot" – it might switch your laptop from UEFI-only to dual mode, although it won't magically make the USB stick itself UEFI-compatible.

Note that "UEFI" and "GPT" are separate things (and neither of them is a 'filesystem', which would be the contents of a partition).

UEFI is the firmware type (like BIOS) with its own boot mechanism, and GPT is just the partition table format (replacement for the DOS "MBR" format).

However, UEFI systems still understand DOS/MBR partitions (there is a defined DOS/MBR "partition type" code for EFI System Partition), and GPT disks still have a "protective MBR" which can contain a BIOS boot sector.