Removing LibreOffice won’t remove the entire GNOME desktop. This used to be the case, but since at least Debian 9 (Stretch), metapackages are handled specially to avoid this scenario. The
gnome package is a metapackage, as indicated by its presence in the “metapackages” section. When such a package is removed for dependency reasons, as happens when you remove the
libreoffice package, all the metapackage’s other dependencies are marked as manually installed, which means they won’t be auto-removed.
You can verify this yourself, as follows. The output you show in your question indicates that
apt will remove
libreoffice and all its dependencies, and the
gnome metapackage but none of its dependencies. Before you do that, run
apt-mark showmanual > ~/manual-pre-removal.txt
libreoffice (it’s safe), and run
apt-mark showmanual > ~/manual-post-removal.txt
Comparing the two files with
meld ~/manual-pre-removal.txt ~/manual-post-removal.txt &
will show that all the other GNOME packages have been marked as manually installed.
Thus to slim down your GNOME installation in Debian, you can remove the packages corresponding to applications you don’t use. As long as you keep
gnome-core installed, you’ll still have a functional GNOME desktop.