Selecting a value from a Gridview Control

[hledman]

I'm trying to get a value from a gridview control and put that value in
a query string. I have a select button on the gridview and once the
user hits the select button i want to capture the the userid. Here's
some example code and you can see where i'm trying to do this in on the
line that starts with Response.Redirect:

protected void SearchUserMenu_MenuItemClick(object sender,
MenuEventArgs e)
{
if (GridView1.SelectedIndex -1)
{
switch (e.Item.Text)
{
case "Security User":
Response.Redirect("~/SecurityUser.aspx?userID=" +
GridView1.SelectedRow.DataItem);
break;
case "WFM User Group/Location":
break;
case "Maintain User":
break;
case "Add User":
break;
}
}
}

any suggestions would be great.
Thanks