Ref:

ASP.Net Nested GridViews – GridView inside GridView with Expand and Collapse feature

Add this below code in Page_Load event.

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        gvCustomers.DataSource = GetData("select top 10 * from Customers");
        gvCustomers.DataBind();
        string script = @"$(document).ready(function () {
            $('img[src*=plus]').trigger('click');});";
        ScriptManager.RegisterClientScriptBlock(sender as Control, this.GetType(), "alert", script, true);
    }
}

Screenshot