Similar question was asked here, but the answer is out of date and I don't have enough reputation to ask in a comment. I know it's possible to boot a Raspberry Pi 4 from USB with the latest firmware update, but does it still need to be on an MBR disk?
Yes, it is possible, I just did it a few hours ago, all with the latest Raspberry Pi OS Lite image.
I just created a GPT on the external disk, a 256MB partition formatted with vfat and labeled boot, and an 64GB one formatted with ext4 and labeled rootfs, just like the SD card. Then I copied all the files from the SD card boot partition into the disk boot partition, and did the same thing with the rootfs partitions. I choose 64GB as a personal choice. I wanted to have a root partition and a data one. You can probably fill the disk with the root partition.
By the way, you will have to update your /boot/cmdline.txt and /etc/fstab files, as your partitions will have different PARTUUID values. Also you have to change the boot order with raspi-config.
I recommend you set up /dev/sdX values in those files and then boot the raspberry and update the values with the right PARTUUID values. I tried to use the PARTUUID values I got when the disk was attached to my laptop, but they are different, for some reason.