First step : Gnome

To install Gnome, I use the group "GNOME Desktop" (to have the list of available list : yum group list )

yum groups install "GNOME Desktop"

Then just have to wait it install the 938 missing packets To make it start the computer with graphical session:

systemctl set-default graphical.target

Install developer tools

There is a group for that too

yum groups install "Development Tools"

Install Guest Addition

To have a better integration with the native system we have to install the VirtualBox "Guest Addition". First to do that we have to connect the carom with the right image, So on "device" menu, choose the last entry : "Insert Guest Addition CD"

Then we will have to mount the cdrom:

mkdir /media/cdrom
mount /dev/cdrom /media/cdrom
cd /media/cdrom

And the execute the script

./VBoxLinuxAdditions.run

And the build failed, when taking a look on the log, it missing a version.h in the kernel headers ... It seems my version of VirtualBox is too old for Centos 7. So I shutdown the VM and virtual box and install VirtualBox 4.2.28

Then restart the installation of Guest Addittions ... and this time it works.