Read the man page for apt-get, it states that -b is for compiling "source packages after downloading them".
The next question is: Why would you want to get and build source when there are binaries already available?
Also bear in mind that "gnome" is just a few kilobytes meta package.
If you want to install gnome then you just need to run:
Code:
apt-get install gnome
(full bloat ~600MB)
Code:
apt-get install gnome-desktop-environment
(smaller ~500MB)
Code:
apt-get install gnome-core gdm3
(even smaller ~370MB)