We have a web application in App Engine. I was wondering whether it is a good idea to move my static resources (i.e Images, CSS files, and JS files) out from App Engine and serv
Neither of these make sense.
1) App Engine has a CDN-like functionality with it's edge cache. Unfortunately, it's not well documented. There's a few Google I/O talks on it. Here's a start: http://eng.pulse.me/backend-tips-the-free-cdn/ There's another talk in Google I/O 2012 that you can find as well. Your static files will be served with the edge cache if they're accessed enough, so no need to move to GCS for this.
2) It's well documented that static files don't take instances. You've already read this, so you 'paranoia' is simply paranoia, and doesn't make sense. It's also very easy to test by uploading a new version onto app engine, accessing a static file, and checking for a new instance in the Admin.