CharlieO you've got yourself some additional training now. It's enough to have to teach them "whoa" and "heel". You've got to teach him this behavior is not acceptable just like you would teach him "here".
You will have to figure out what punishment works best. GSP's don't require the strong hand that English Pointers do as you well know. My girl likes to tip over the water bowl when she's upset. If she sees me and I don't let her out of the pen immediatly she gets mad and takes it out on the waterbowl. I couldn't have that behavior because I work 12 hr.shift. I couldn't leave and let her turn over the water and be without water for 13-14 hrs.
I like to use the waterhose to discourage unwanted pen behavior i.e. digging, barking, destroying the fence/gate. The trick with my current GSP was she would see/smell me coming with the hose and stop misbehaving. Well you can't blast them when thier not misbehaving or they won't get it, right? So I rigged the waterhose to the pen with it aimed at the waterbowl. This way I could sneak to the spigot when she started and blammo and then holler "NO". This should work for you. You may have to set up the hose outside the pen to avoid the dog chewing the hose.
Just some ideas. Keep us posted on your progress.