Hi,
You could put EnableTheming="false" on your GridView then I think it will use the properties you specified.
Alternatively you could set those properties on the GridView item in the theme.skin file of your skin.
However, if you are hosted in Medium Trust then it won'tuse the theme.skin file from your skin folder but will instead use the one from App_Themes/Default/theme.skin
Note that there are a number of CssClass settings on the GridView as you will see in the them.skin file and it is preferable to use Css classes rather than style properties because when you use style properties they render as inline styles and bulk up the markup as explained here.
http://www.mojoportal.com/understanding-the-theme-skin-file.aspx
To view the markup rendered by the update panel you could use the Firebug plugin for Firefox which allows you to inspect the html after the javascriopt has modified it.
Hope it helps,
Joe