Index is out of range

<asp:GridView ID="gvtest1" runat="server" AutoGenerateColumns="False"
            style="z-index: 1; left: 68px; top: 60px; position: absolute; height: 105px; width: 390px; background-color: #ECE9D8;"
           DataSourceID="SqlDataSource1"
            HorizontalAlign="Center" BorderColor="Maroon" ForeColor="#CC9900">
         <Columns>
<asp:TemplateField>
<ItemTemplate>
<asp:CheckBox ID="chkSelect" runat="server" />
</ItemTemplate>
</asp:TemplateField>
</Columns>

protected void btnProcess_Click(object sender, EventArgs e)
  {

      string str = string.Empty;
      string strname = string.Empty;
      string v_amt = string.Empty;

      foreach (GridViewRow gvrow in gvtest1.Rows)
      {
          CheckBox chk = (CheckBox)gvrow.FindControl("chkSelect");
          if (chk != null & chk.Checked)
          {
              //str += gvtest.Rows[gvrow.RowIndex].Value.ToString() + ',';
              str += gvtest1.DataKeys[gvrow.RowIndex].Value.ToString() + ',';
              strname += gvrow.Cells[2].Text + ',';
              v_amt += gvrow.Cells[5].Text + ',';
              string v_stringamt = gvrow.Cells[5].Text;
              int v_amt1 = Convert.ToInt32(v_stringamt);

              v_totamt = (v_totamt + v_amt1);
          }
      }