I do not recall the gates being locked. If they were someone would have to be posted for emergencies. I recall the last time I was camping (primative) we made a few night runs (well into 2 or 3 am). Maybe that is only the nice camp spots?
supposedly arrowhead, badin lake campground lock at 10pm and reopen at 6am according to the signs. Signs say if you need in or out during those times, you have go to the camp host in whatever site. That's what the signs say. In real life, I have never found them closed and locked at night.
IIRC there are no gates to the campgrounds. The only gates I do know of are at Kings Mtn Point which is a daytime use area only. They do close them at 10pm like someone already mentioned. The rest of the camp grounds, as far as I know, are 24hr.
I went out on Daniel trail at night with a group of Jeeperz last July and we left the group camp at around 10pm I believe. I didn't leave the forest until about 1am to head home.
I've stayed at Arrowhead many times and never had the gates closed on me. We've always been able to go on night runs and if people come late have always been able to get in.