I was trying to resize the root partition for RaspBSD 12 on a Raspberry Pi 3 and ran in some trouble, since growfs showed "operation not permitted".
Here was my solution to resizing:
This showed that my disk was named mmcsd0 and that my root partition had the index "2".
gpart resize -i 2 -a 4k -s 20gb mmcsd0
Here I'm resizing the root partition, 2, to 20gb
save the changes
service growfs onestart
resize the file system