To set up RAID 10 first create two or more mirrors from identical disks:

gmirror label -v -b round-robin md1 da0 da1
gmirror label -v -b round-robin md2 da2 da3

Next stripe the mirrors:

gstripe label -v -s 131072 md0 /dev/mirror/md1 /dev/mirror/md2

After that you will need to create a file system:

newfs /dev/stripe/md0