Hello,
please add datatable into session during Gen_Lic_Grid loading on page_load event and then use it as below :
protected void Gen_Lic_Grid_PageIndexChanging(object sender, GridViewPageEventArgs e)
{
Gen_Lic_Grid.PageIndex = e.NewPageIndex;
Gen_Lic_Grid.DataSource = Session["LicList"];
Gen_Lic_Grid.DataBind();
}