A basic grid in Sencha GXT3 that displays JSON data can be found in in https://github.com/prabhu-durasoft/JSONGridGXT3.
The JSON data that is loaded using HttpProxy class is shown below.
{ "countries" : [ { "name": "India" , "capital" : "New Delhi"}, { "name": "USA" , "capital" : "Washington, D.C"}, { "name": "UK" , "capital" : "London"} ] }