There is link to https://wiki.archlinux.org/title/Systemd#Editing_provided_units, there it is explained. Read it carefully.
Here you go:
sudo systemctl edit --full gdm
(this will create a new alternative service file. The original file will not be touched)
There you add between [Service] and ExecStart=/usr/bin/gdm: ExecStartPre=/bin/sleep 2 like this:
[Service]
ExecStartPre=/bin/sleep 2
ExecStart=/usr/bin/gdm
and save it with CTRL+S and close it with CTRL+X.
sleep 2 stands for “wait 2 seconds”. Add more time if needed.
OR
add change the type to idle:
[Service]
Type=idle
ExecStart=/usr/bin/gdm
(this will wait not more than 5 seconds)
You can revert it any time with:
sudo systemctl revert gdm