Dear Debian maintainer of gdm3,
there is a problem with gdm3 on a fresh buster netinstall. If multiple keyboard layouts are
selected, e.g., de,us or fr,us, in /etc/default/keyboard like
# KEYBOARD CONFIGURATION FILE
# Consult the keyboard(5) manual page.
XKBMODEL="pc105"
XKBLAYOUT="de,us"
XKBVARIANT=""
XKBOPTIONS=""
BACKSPACE="guess"
then gdm3 will always start with the us layout even if de or fr is present first.
However, XKBLAYOUT="de,fr" or XKBLAYOUT="fr,de" work as expected and GDM3, respectively,
starts with a german or french layout.