There isn't any package named
gnome. I think you meant
Either your system does not have proper packages list. Do a
sudo apt-get update
to check if everything is in order
If the problem still comes, check what all are contained in the package. Chances are high that it is a
metapackage. It means that a package which has multiple packages as it's dependencies.
Meta-packages don't contain anything. If you want to install say kubuntu on already existing ubuntu installation then all you need to do is to install
kubuntu-desktop which will take care of the packages which need to be installed.
Now if you remove any single of the package which are dependency of that meta-package, then that meta-package is shown to be removed.
Don't worry, your whole gnome is not going to be removed. Only the meta-package is going to be unchecked as the dependency requirement for that package is not fulfilled.
I am not very much sure what the problem you are getting. Probably the solution I explained might not be the solution to your problem, nevertheless I thought it is good to explain what all problems can exist with the same symptoms.