protected void GridView1_PageIndexChanging (object sender, GridViewPageEventArgs  e)
{
    GridView1.PageIndex = e.NewPageIndex;
    bindGridView(); //bindgridview will get the data source and bind it again
}

private void bindGridView()
{
     GridView1.DataSource=getData();
     GridView1.DataBind();
}

    protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
    {
        GridView1.PageIndex = e.NewPageIndex;
        bindGridView(); //bindgridview will get the data source and bind it again
    }

    protected void Page_Load(object sender , EventArgs e)
    {
        if(!IsPostBack)
         bindGridView();

    }
    //this is some sample data 
    private void bindGridView()
    {
        DataTable t = new DataTable();
        t.Columns.Add("Col1");
        t.Columns.Add("Col2");
        DataRow r = null;
        for (int i = 0; i < 25; i++)
        {
            r = t.NewRow();
            r.ItemArray = new object[] { "Val" + i, " Another " + i };
            t.Rows.Add(r);
        }
        GridView1.DataSource = t;
        GridView1.DataBind();
    }

<asp:GridView OnPageIndexChanging="GridView1_PageIndexChanging" AllowPaging="true" PageSize="10" ID="GridView1" runat="server" AutoGenerateColumns="true">