Label1.Text = ((CheckBox)GridView1.SelectedRow.Cells[1].Controls[0]).Checked.ToString();
or
try
protected void CheckBox_Checked(object sender, EventArgs e)
{
CheckBox chkbox = (CheckBox)sender;
GridViewRow r = (GridViewRow)chkbox.NamingContainer;
if(chkbox.Checked)
{
}
}