try like below
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
DataTable dt =GetTable();
GridView1.DataSource = dt;
GridView1.DataBind();
ViewState["Table"] = dt;
}
}
protected void Button1_Click(object sender, EventArgs e)
{
int index = GridView1.SelectedIndex;
DataTable dt = (DataTable)ViewState["Table"];
DataRow row = dt.NewRow();
dt.Rows.InsertAt(row, index);
dt.AcceptChanges();
GridView1.DataSource = dt;
GridView1.DataBind();
ViewState["Table"] = dt;
}