gridx and dojox.grid.DataGrid are not the same thing. dojox.grid.DataGrid is deprecated and only works with legacy dojo/data stores. You may use dojo/data/ObjectStore
to wrap a dojo/store store with the dojo/data API:
var teststore = new ObjectStore({ objectStore: new Store({ … }) });
However, if you are starting a new project, you should use dgrid instead, which works natively with dojo/store stores.