Hi iammann,
Check this example. Now please take its reference and correct your code.
For typing in hindi you need to download the font from the link below.
https://www.ffonts.net/Kruti-Dev-010.font.download
Then copy the font in the project folder to use the font in your code.
HTML
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <style type="text/css">
        @font-face
        {
            font-family: 'HindiFont'; /* Font Name to use*/
            src: url('<%= ResolveUrl("~/Fonts/k010.ttf") %>'); /* Font path*/
        }
        body
        {
            font-size: 20px;
        }
        .hindiFont
        {
            /* Assign the font*/
            font-family: 'HindiFont';
        }
    </style>
    <script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
    <script type="text/javascript">
        $(function () {
            $.each($('#GridView1 tr'), function () {
                $(this).find('[id*=txtText]').attr('class', 'hindiFont');
            });
        });
    </script>
</head>
<body>
    <form id="form1" runat="server">
    <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false">
        <Columns>
            <asp:BoundField DataField="Id" HeaderText="Id" />
            <asp:BoundField DataField="Name" HeaderText="Name" />
            <asp:BoundField DataField="Country" HeaderText="Country" />
            <asp:TemplateField HeaderText="Comments">
                <ItemTemplate>
                    <asp:TextBox runat="server" ID="txtText" />
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>
    </form>
</body>
</html>
Code
C#
protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        System.Data.DataTable dt = new System.Data.DataTable();
        dt.Columns.AddRange(new System.Data.DataColumn[3] { 
                        new System.Data.DataColumn("Id", typeof(int)),
                        new System.Data.DataColumn("Name", typeof(string)),
                        new System.Data.DataColumn("Country",typeof(string)) });
        dt.Rows.Add(1, "John Hammond", "United States");
        dt.Rows.Add(2, "Mudassar Khan", "India");
        dt.Rows.Add(3, "Suzanne Mathews", "France");
        dt.Rows.Add(4, "Robert Schidner", "Russia");
        GridView1.DataSource = dt;
        GridView1.DataBind();
    }
}
VB.Net
Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Dim dt As New System.Data.DataTable()
        dt.Columns.AddRange(New System.Data.DataColumn() {
                            New System.Data.DataColumn("Id", GetType(Integer)), _
                            New System.Data.DataColumn("Name", GetType(String)), _
                            New System.Data.DataColumn("Country", GetType(String))})
        dt.Rows.Add(1, "John Hammond", "United States")
        dt.Rows.Add(2, "Mudassar Khan", "India")
        dt.Rows.Add(3, "Suzanne Mathews", "France")
        dt.Rows.Add(4, "Robert Schidner", "Russia")
        GridView1.DataSource = dt
        GridView1.DataBind()
    End If
End Sub
Screenshot
