I am writing a web application using GWT and App Engine. My application will need to post and query items based on their latitude, longitude.
As a result of google\
An alternative to do geo spatial searches in App Engine is the Search Api. You won't need to worry about geohashing or implementation details, and you'll be able to search for elements close to geo point.
https://developers.google.com/appengine/docs/python/search/overview#Performing_Location-Based_Searches