I agree about not leaving the dog outside unattended -- a determined GSD will find a way out. I once saw one actually dismantle part of a wood privacy fence, one board at a time, to get out!
If your dog is fast enough to slip out even when you're out there due to low spots in your yard, here's a trick we recommend during rescue application home checks when we spot gaps between the ground and fencing:
You can cheaply fill that gap between the ground and fencing with long steel spikes (several feet long, at HD or Lowes) -- drive them deeply into the ground using a sledge hammer, at 6" intervals. They may stick up 6" or so. It's not particularly pretty, but it works for most dogs. When you put them in at 6" intervals, you create a steel cage around your fence line.