Great, simple article on gmirror, thank you! Super helpful compared to other stuff out there.
One improvement I'd suggest - where I stumbled a little - I have SATA drives ad4 and ad6, so where you say:
"Change each ad to a gm, and insert a mirror after /dev. For example, /dev/ad4s1a becomes /dev/mirror/gm0s1a. Unless you've made extra partitions, you'll have ad4s1 devices ending in a, b, d, e, and f and will need to edit each of those lines."
...I needed to change e.g., ad4 to gm0 ... I just replaced ad with gm and rebooted and got stuck for a bit in "mountroot>" purgatory :)