Don't use sysinstall after the system is installed. sysinstall should be used only for installing the OS. Once that's done, use the OS tools for everything and ignore sysinstall completely.
A google search for "gmirror how-to freebsd" turns up a lot of hits. Here are a couple of the nicer ones:
http://www.onlamp.com/pub/a/bsd/2005...SD_Basics.html
http://lantech.geekvenue.net/chuckti...464/index_html