/usr/share/gdm/locale.alias is in gdm3 3.28.0-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | # You could insert none UTF-8 locales likes C, ja_JP.eucJP
# The format is language label, space and locale name but
# the language label is no longer used.
#
# This file will be removed in the future once gdm setup tool is generated.
#
#Unspecified C,POSIX
|