If you want to run the update in a cron-job, you should use the flag -dy.
With this flag, the new packages will be downloaded but not installed.
You may know some software like Firefox, GIMP, etc, but what they really are is just lots and lots of files that have been compiled into one.
The people (or sometimes a single person) that write this software are known as upstream providers, they compile their code and write up how to get it installed.
On the other hand, you can get rid of these leftovers by calling purge even on already removed packages.
Note that this does not affect any data or configuration stored in the users' home directory.
Following the upgrade instructions found in the release notes is the best way to ensure that your system upgrades from one major Debian release to another (e.g. Removing a package removes all packaged data, but leaves usually small (modified) user configuration files behind, in case the remove was an accident.
The requested action can be overridden for specific packages by appending a plus ( ) to the package name to install this package or a minus (-) to remove it.
A specific version of a package can be selected for installation by following the package name with an equals (=) and the version of the package to select.
Before the GNOME install process, first read and follow the instructions in the Xorg guide to setup a X environment.
X is the standard base for all desktop environments in Linux.