If you installed the metapackage "gnome" using aptitude, then all of its
dependencies would be marked as having been automatically installed.
Whenever you remove a package using aptitude, it will check the
dependencies of that package. If a) they are marked as having been
automatically installed, and b) no other installed package also depends on
it, then they will also be removed.
Thus, if you have not otherwise indicate you desire the dependencies of the
"gnome" metapackage (you can toggle automatic-status within aptitude on a
per-package basis, or just manually mark one or more of the dependencies
for installation) and the gnome metapackage's dependencies are no longer
being fulfilled (because you've tried to remove tomboy, or whatever), the
rest will go as well.
This is potentially confusing for people who type "aptitude install gnome"
and subsequently remove a dependency; however, I don't think in practise
this can be that many people. Most people will obtain GNOME by default when
they install Debian and select "desktop environment". This method does not
install the "gnome" metapackage (nor tomboy).
I understand that newer versions of apt itself also have this automatic-
dependency tracking, but I don't think the one in Debian stable does.