How to display selected data(using checkbox) from one gridview of one page to another gridview of another page?

Upon button click, create a new DataTable (called "dtSelected") with columns to store the data from checked rows of the first data table. Then you have 2 options:
1. Store this "dtSelected" in a session variable, followed by a Response.Redirect to the new page (called "GridViewPage2"), like this

Session["dt"] = dt;
Response.Redirect("GridViewPage2.aspx");

or
2. Store this "dtSelected" in a Context.Item collection then do a Server.Transfer to the new page, like this:

Context.Items.Add("dt", dt);
Server.Transfer("GridViewPage2.aspx");

Next, on the Page_Load of the new page, do something like this:

if (!this.IsPostBack)
{
    /*
    // option 1
    if (Session["dtSelected"] != null)
    {
        gvSelected.DataSource = Session["dtSelected"];
        gvSelected.DataBind();
    }
    */
    // option 2
    if (Context.Items["dtSelected"] != null)
    {
        gvSelected.DataSource = (DataTable)Context.Items["dtSelected"];
        gvSelected.DataBind();
    }
}

Which option to adopt? Decide for yourself after reading Difference Between Response.Redirect() and Server.Transfer() Methods in ASP.NET[^].