protected void gvRegs_PageIndexChanging(object sender, GridViewPageEventArgs e)
        {
            gvRegs.PageIndex = e.NewPageIndex;
            DataBind(gvRegs);
        }

// получаю так gvRegs.DataKeys[gvRegs.SelectedIndex].Values[1]

  gvRegs.DataSource = KB.GetTaskRegs(gvTasks.DataKeys[gvTasks.SelectedIndex].Value);
  gvRegs.DataBind();

with cte as
(
	select row_number() over(partition by ac,tasknumber order by date desc) n
		,ac
		,tasknumber
		,date
		,fh
		,fc
	from task_reg
)
select rp.RegistrationNumber
               ,rp.id id
		,c.*
 from rplane rp
left join cte c on rp.id=c.ac 
and c.n=1 
and c.tasknumber=27470