Electri collars are now illiegal in Wales and Scotland and should never ever ever be used.
If you think it is not as bad as it sounds put it on your wrist and get someone to operate it.
As a dog behaviourist I would not recommend using an electric collar. On an already nervous dog it will make thinhgs 100% worse.
If you can not afford the advice of a behaviourist (although it may be a long term investment).
I am assuming a lot here and without seeing the dog do not feel entirely happy giving advice but usually a dog that bolts is an anxious dog. If a dog runs it releases endorphins with makes it feel happier and less anxious. Hence the need to bolt when it gets the chance.
So I think you need to aproach this from many angles.
GSD are highly intelligent dogs and need things to do. So up the general training. Use a clicker and train tricks, a good stay, a good sit, wait, leave it, watch me, find, retrieve, drop etc. (Mary Rays book Called Dog Tricks is a great book telling you how to do tricks using a clicker).
By keeping the dog busy the anxiety levels will drop.
Back to the recall. Put the dog in another room in the house and prepare the dogs meals. Get a dog whistle (Acme 2.11) and blow the whistle 3 times and open the door to let the dog have the meal. DO this at every meal time and soon the whistle will mean great things happen.
After a few days put the dog in another room and again blow the whistle three times and give a treat.
Next let someone hold the dog in the garden and then blow the whistle while you are inside they should release the dog and hopefully come to you.
If at any stage the dog does not come instantly go back a stage.
After a while you will be able to build up bigger distractions in the house. eg the person in the garden holding the dog has a sausage but the dog will come to you who blows the whistle.
Only when the recall is solid try at the park etc but at this point do so on a long lease. This however will take a while to get to.
All the time you are doing this you need to be training like mad 4 or 5 times a day minimum for a few minutes each time. This will help to build the dogs confidence and bring down the anxiety and the need to run will be less.
The more exercise you can give the better -have you thought about joining an agility class or maybe booking swimming sessions. All of this will help to make the dog more confident and less likely to run.
Dog training classes are usually pretty cheap and it would give you the chance to practice recall in a secure environment. If you don't practice it you will never have a dog that recalls.
This is not something that will change overnight and will need a lot of dedication and time on your behalf
Good luck