Resizing the root partition in RaspBSD 12

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:
gpart show
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

gpart commit
save the changes

service growfs onestart
resize the file system

Sources:
FreeBSD.org documentation
DigitalOcean