/etc/cobbler/pxe/gpxe_system_linux.template is in cobbler-common 2.4.1-0ubuntu2.
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 | #!gpxe
kernel http://$server:$http_port/cobbler/images/$distro/$kernel_name
imgargs $kernel_name $append_line
initrd http://$server:$http_port/cobbler/images/$distro/$initrd_name
boot
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