@Override
public void surfaceCreated(SurfaceHolder holder) {
  if(gt == null) {
    gt = new GameThread(getHolder(), this);
    gt.setRunning(true);
    gt.start();
  }
}

public void onPause(){
  ... other stuff
  if(gt != null){
    try {
      gt.setRunning(false);
      gt.join();
      gt = null;
    }catch(Exception e){
      e.printStackTrace();
    }
  }
}