Here is a thought, it's certainly going to take time and is not 100% bullet proof but GSD are very smart!
There is the possibility that you could train her to respect a "portable boundary" flags or boxes make a big square put her inside and make it clear to her that she is "not" to exceed the boundaries!
"On the lawn" is my command for my guy. I always used it with my guy on our front yard. I did not realize it was a :command till one day on a walk returning home, I unclipped him and said "on the lawn" I watched stunned as he "wobbled" (disability) down the side walk and onto our front yard, three houses away!!
I was stunned...hey I'm better than I thought!
Just an idea not a recommendation!