This is the GWT forum, so I'd suggest posting GAE questions on the GAE forum instead. That's here:
https://groups.google.com/forum/?fromgroups#!forum/google-appengine
Having said that, if you have a standard Java app, it's likely not going to "just work" on App Engine, which comes with lots of restrictions and platform-related gotchas. App Engine doesn't have a goal of being a standard J2EE stack.
If your app is already built and you just need somewhere to put it, use a normal VM-based hosting service. From your other question it looks like you want something free, which is also unlikely. Unless you want to do a lot of re-development (which will likely cost a lot more than the hosting fees), I'd suggest paying for the hosting while you test your proof of concept.