Replace your two radio buttons with radiolist as below
<asp:radioButtonList ID="rbtlistdrawings" runat="server" RepeatLayout="Flow" RepeatDirection="Horizontal">
</asp:radioButtonList>
and put this piece of code in page_load evevt
if (!IsPostBack)
{
rbtlistdrawings.Items.Add("Draw Polygon");
rbtlistdrawings.Items.Add("Craw Circle");
}
lastly change the Gmap_click1 event as below
protected string GMap1_Click1(object s, GAjaxServerEventArgs e)
{
if (rbtlistdrawings.SelectedIndex == 0)
{
return "polygon clicked";
}
else
{
return "Circle clicked";
}
}