Just delete /tmp in Gparted then create a new partition in the free space called /whatever_you_like
Change your /etc/fstab to reflect that change. There should be an entry for /tmp partition in there and just change /tmp to whatever you called your newly created partition. You may need to check you have the right UUID in there. You can do that with:
... in a terminal.
Delete the mount point /tmp from /media/tmp and replace with one for the new partition and you should be good to go.
sudo mkdir /media/whatever_you_called_it