Control ‘GridView’ of type ‘GridView’ must be placed inside a form tag with runat=server.

Please add below functions in code-behind cs file.

  public override void VerifyRenderingInServerForm(Control control)
{
/* Confirms that an HtmlForm control is rendered for the specified ASP.NET
server control at run time. */
}

public override bool EnableEventValidation
{
get { return false; }
set { /*Do nothing*/ }
}