if you want to download the main gnome and then build it from there you are ok to do that and waist your time
but if you have already a version of linux like ubuntu or mint or … you can see how we do it. you can see the main document here. In this example we will build the 64 bit kde version

gedit /etc/apt/sources.list

and add these lines:

deb http://ftp.it.debian.org/debian wheezy main                                
deb-src http://ftp.it.debian.org/debian wheezy main                            
deb http://security.debian.org/ wheezy/updates main                            
deb-src http://security.debian.org/ wheezy/updates main 

deb http://http.kali.org/kali kali main non-free contrib
deb-src http://http.kali.org/kali kali main non-free contrib
deb http://security.kali.org/kali-security kali/updates main contrib non-free
deb-src http://security.kali.org/kali-security kali/updates main contrib non-free

save it and the do

apt-get update
apt-get install git live-build cdebootstrap kali-archive-keyring
git clone git://git.kali.org/live-build-config.git

then edit the live-build-config/config/package-lists/kali.list.chroot
and when you see this:

# Graphical desktops depending on the architecture
#
# You can replace all the remaining lines with a list of the
# packages required to install your preferred graphical desktop
# or you can just comment everything except the package of your
# preferred desktop.

delete every thing after and add this:

#if ARCHITECTURES i386 amd64
# *** GNOME DESKTOP ***
kali-defaults
kali-root-login
desktop-base
kde-plasma-desktop
#or if laptop
kde-plasma-notebook
#endif

#if ARCHITECTURES armel armhf
# *** XFCE DESKTOP ***
xfce4
network-manager-gnome
network-manager-kde
network-manager
#endif

then

cd live-build-config
dpkg --add-architecture amd64
lb config
lb config --architecture amd64
lb build

befor doing lb build make sure that you have some time to wait for it and a internet connection because it will build the iso for you.
done! you will have your iso if nothing goes wrong