sorting on category field in gridview

Refer to the following posts, hope it helps

http://code.msdn.microsoft.com/office/Ejemplos-sobre-cmo-usar-el-69cd5f16

http://www.aspdotnet-sharepoint.com/2011/12/how-to-sorting-in-gridview.html

http://stackoverflow.com/questions/7577353/gridview-sort-is-there-a-better-way-to-perform-a-gridview-sort-in-asp-net-for

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--Cheers