Have been guilty of cheeky shortcuts on the firing range just north of there. General rule is Sunday’s are OK, you are supposed to ask permission,they put up red flags too. Wathgill Camp is bit closer to Catterick,tank country I think.
Re camping, there are plenty of unclassified roads to the north of Gunnerside area, many that turn into bridleways. Also a few carparks: http://tdf.roadworks.org/
Might have a recky for signs this weekend.