I had two OSes, I tried to delete Windows and keep the Ubuntu.
I tried to delete my Windows partition to have some extra space in my Ubuntu drive. After deleting the partition for the Windows I can't use that space. That is still showing unallocated when ever I am trying to access that it's not happening.
To use that unallocated space, you can create a new partition from that (right-click on it).
Or you could extend the other partition (sda6 in your case) using the Resize/Move option.
If you have installed your Ubuntu with LVM, you could create an extra partition and add that to the LVM volume or you can boot you computer with a gparted live cd/usb and resize you partition the gui way.