The box is checked, but when you type /gbk it says "type /help blah blah"
That's not right. Does it say "Out of Date" in the Addons box or anything like that? If the mod is loaded, then typing /gbk should work. If not, try disabling other mods and see if that changes things.
Depending on how many mods your running you could also be running out of addon memory. I always increase mine to like 128. I think the default is like 32 or 64.
