GPT is good, that means you're not limited to only 4 partitions and can create many more. But the bigger question is, is there any free space on the disk? If not then you will most likely have to resize the last partition on the disk to make room for FreeBSD. How you do this is a more a Linux question and is going to depend if you used LVM, ext4, xfs or any of the other formats you could have used. The FreeBSD installer isn't going to resize that partition for you, you will have to do this yourself first.