/usr/share/live/images/gnome-desktop/config/bootloaders/isolinux/isolinux.cfg is in live-image-gnome-desktop 4.0~a5-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | include menu.cfg
default vesamenu.c32
prompt 0
timeout 0
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