You accept gzip and deflate responses, so this "gibberish" is actually the compressed response. That's not a problem though, you can tell HttpWebRequest to automatically decompress it:
request.AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate;
I see you also accept "br", and I couldn't find auto-decompression for that, so it's probably a good idea to remove "br" from your accepted encodings, unless you want to take care of the decompression yourself if you happen to get a "br" response.