Great that you've learned things, but nothing you've mentioned there is specific to GAE. Many people learned abour sharding and such well before GAE even existed (indeed, perhaps even before Google existed).

This is akin to saying "Rails made me a better developer", which in and of itself is not bad or wrong, but isn't really terribly useful as a datapoint when deciding which framework to choose. Frameworks by their very nature (almost always) force concepts on you that improve your code. So too with GAE - by limiting some of what you can do, they can provide a more focused service.