GParted won't let me edit a partition

I have this 72 GB hard disk that I installed Ubuntu 14.02 LTS 64 bit on. Now, I want to split it and leave 50 GBs for Ubuntu and install in the to-be-created partition the server version of the OS.

The issue at hand is that GParted won't let me change the size of the partition I want to shrink. It reads : minimum size = 72222 mb , maximum size = 7222 mb and obviously doesn't allow me to edit the partition.

Firstly, you can not work with mounted partitions. Since you are trying to modify the partition on which Ubuntu is installed, you can't do this from your installed OS as you cant unmount the partition. So what you have to do is:

1. Create a live USB/DVD and boot into Ubuntu Live
2. In Gparted you start from the live environment, unmount the partition. If the partition is mounted, 'key' icon will be present next to the name of the partition. To unmount from Gparted, just right click on the partition and click Unmount.
3. Now modify the partition.
4. Reboot and Voilà! It's done.