I've figured it out
the keywords were: get the packagename from the deb's [gdm_XXX.deb etc], with dpkg -I, result: "gdm", and then eg.:
apt-rdepends gdm -s --show=DEPENDS > list
and then download the dependencies with "aptitude download list line1" etc, and it will not download the eg.: x-terminal-emulator because it's not a package
my little "dependency searcher" script is done, I just have to test it now
thank you! :D