You can try something like this
protected void Submit_Click(object sender, EventArgs e)
{
foreach (GridViewRow row in Gridview1.Rows)
{
CheckBox cb = (CheckBox)row.FindControl("CbID");
if (cb != null && cb.Checked)
{
var selVal= GridView1.DataKeys[row.RowIndex].Value;
lblSelected.Text += Convert.Tostring(selVal);
}
}
}
Then Add a panel with a modal popup which consists of a control ( here just i have used label ) to display the selected values in it and associate the modal popup targetid with the Submit button . Add a simple javascript if you want to use it that way.
Hope this helps