We have two adopted dogs, and found that they both had a tendency to bolt/run away in the first few months.
I suggest when outside keep them supervised on a leash/check cord at all times in for the beginning until they can be trusted, even in off leash areas/country (this is where ours would bolt). It's not fun looking for a dog running down the highway, it's happened to us.
GSP are very smart dogs with strong memory, we found ours took about a year to adjust to our home, and was rather traumatized by the sudden change/move in the beginning. What I am saying is that it will take time - I wish we had taken it more easy on ours for the first couple of weeks and to just let him relax and just be.
Our second adopted one had a tendency to want to run out the front door too. We got her when she was 7. I slammed it hard and loud in front of her a few times and she learned quick. Now we have progressed to saying sit, having her wait/continue the sit for the door to open, continuing the wait while the door is open, then "okay" to go through the door. If she moves out of a sit/wait, I close the door immediately, which in itself corrects the dog because they realize they can't go outside.
Shock collars are for correcting a known command. I believe they do work in communicating/correcting your dog from a distance for a known command. I believe they can create suspicious dog that does not trust you if used incorrectly. Teach what you want from the dog first before correcting.