Craft hosted on Google App Engine - failing to load Craft classes?
I got the same error and started debugging with XDebug.

I've put some hours in this and my conclusion is: You can't run Craft on GAE. It gets worse: You can't even run Yii1 (the framework Craft was built on) on GAE. So that's where it stops.

I did manage to get Craft running on a GAE development server instance, but to do that I had to uncomment every filesystem check in both Craft and Yii (functions like is_dir(), etc). These functions don't even work with the gs:// wrapper.