How to determine the hexadecimal encoding of GPIO?

yes, your understanding is correct.

may I also know which Jetpack release you’re using?
please see-also Cboot in 32.7.2 fails to read extlinux.conf - #15 by WayneWWW for the bug fixes if you’re using l4t-r32.7.2
please check release tag, $ cat /etc/nv_tegra_release for confirmation.
thanks