Sorting on GridPanels is done by either clicking on the column header, or by using the Sort option in the column menu.
Sorting therefore involves knowing two pieces of information—the column you want to sort, and the direction to sort in.
Tip
Sorting by multiple fields is supported by the Store but not the GridPanel.
Natively, the GridPanel itself doesn't support the ability to sort by multiple columns. The underlying Store, however, does.
Given that the GridPanel uses a Store, you can build a custom implementation to sort by more than one column. It would likely require a lot more Ext JS JavaScript to be written. An example is provided in Ext.NET's Examples Explorer at http://examples.ext.net/#/GridPanel/Paging_and_Sorting/Multiple_Sorting_Local/.
Like paging and filtering, sorting can also be done either client side or server side.