I rarely use gnome-software, and on most boot-ups it is useless. However there is not an entry at start-up apps, not even after showing hidden startup apps (https://help.ubuntu.com/community/ShowHiddenStartupApplications). Also, I haven't found any gnome-software-related process from systemd-analyze to disable it with systemctl.
/etc/xdg/autostart/ and remove the
NoDisplay=true line in
gnome-software-service.desktop file (or change
false). Then GNOME Software should appear in Startup Applications list. You can disable it from there.
If GNOME Software still doesn't appear in Startup Applications list following the suggestion above, first copy of the
/etc/xdg/autostart/gnome-software-service.desktop file to
~/.config/autostart and then append an
line to the copied file.
From https://ask.fedoraproject.org/en/question/92941/how-to-stop-fedora-24-gnome-software-auto-updating/, try these two options:
Disable it in gsettings
gsettings set org.gnome.software download-updates false
If that fails .. mask (disable) the backend service, PackageKit, the service used by Gnome Software.
sudo systemctl mask packagekit.service