call a javascript function onload or onsorted of gridview

You may follow below steps for this.

1) Add a reference of your External JavaScript file in your Aspx Page.

<script type="text/javascript" src="Scripts/MyJavascript.js"></script>

2) Use Load or/and Sorting Evetnts of your GridView to call your JavaScript function.

http://msdn.microsoft.com/en-us/library/system.web.ui.control.load.aspx

http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.gridview.sorting.aspx

3) Call JavaScript function as below.

this.ClientScript.RegisterClientScriptBlock(this.GetType(), "GridLoad", "<script type="text/javascript">Test()</script>");

All you need is some customization(by your own), here some examples for you.

Using JavaScript with ASP.Net GridView Control[^]
GridView Examples in ASP.NET 2.0 3.5[^]
GridView Examples[^]