@DOCTORPANGLOSS you saved me! Thank you!!

I just want to point out that:

1. You say it's documented, and provided a link, and the letters "gzip" do not appear in your link, so clearly the behavior that everyone's observing with gzip only being supported on some platforms is not documented.
2. On your latest scripting runtime, there is gzip compression written in C#:

UnityWebRequest www;
string text;
if (www.GetResponseHeader("Content-Encoding") == "gzip"
&& Application.platform == RuntimePlatform.WindowsPlayer)
{
// GZipStream is from System.IO.Compression in .Net 4.5+
var stream = new StreamReader(new GZipStream(new MemoryStream(www.downloadHandler.data),
CompressionMode.Decompress));
text = stream.ReadToEnd();
stream.Close();
}
else
{
text = www.downloadHandler.text;
}

// Do what you need to do with the text
Debug.Log(text);

// Don't forget to dispose the download handler
www.downloadHandler.Dispose();

Or maybe you can have a callback that letting developers to handle content-encoding which you can not handle.

Ok but some CDN server send Content-Encoding back with gzip even you did not set Accept-Encoding and UnityWebRequest.GetAssetBundle will go wrong.