<%@ Application Language="C#" %>
<%@ Import Namespace="System.IO" %>
<%@ Import Namespace="System.IO.Compression" %>

<script runat="server">
void Application_PreRequestHandlerExecute(object sender, EventArgs e)
{
    HttpApplication app = sender as HttpApplication;
    string acceptEncoding = app.Request.Headers["Accept-Encoding"];
    Stream prevUncompressedStream = app.Response.Filter;

    if (!(app.Context.CurrentHandler is Page ||
        app.Context.CurrentHandler.GetType().Name == "SyncSessionlessHandler") ||
        app.Request["HTTP_X_MICROSOFTAJAX"] != null)
        return;

    if (acceptEncoding == null || acceptEncoding.Length == 0)
        return;

    acceptEncoding = acceptEncoding.ToLower();

    if (acceptEncoding.Contains("deflate") || acceptEncoding == "*")
    {
        // defalte
        app.Response.Filter = new DeflateStream(prevUncompressedStream, CompressionMode.Compress);
        app.Response.AppendHeader("Content-Encoding", "deflate");
    }
    else if (acceptEncoding.Contains("gzip"))
    {
        // gzip
        app.Response.Filter = new GZipStream(prevUncompressedStream, CompressionMode.Compress);
        app.Response.AppendHeader("Content-Encoding", "gzip");
    }
}
</script>

Gzip example in Global.asax

C#