Does pyexiv2 support writing XMP tags to GIFs?

I tried adding an XMP tag to a gif, but I received the following error:

Traceback (most recent call last):
  File "exif.py", line 19, in <module>
    metadata.write()
  File "/usr/lib/python2.7/dist-packages/pyexiv2/metadata.py", line 119, in write
    self._image._writeMetadata()
IOError: Writing to GIF images is not supported

This is my code:

#!/usr/bin/python
# -*- coding: utf-8 -*-

import os
import pyexiv2

metadata = pyexiv2.ImageMetadata("smug.gif")
metadata.read()

key = "Xmp.xmp.Keywords"
values = "gif"
metadata[key] = pyexiv2.XmpTag(key, values)

tag = metadata[key]
metadata.write()

Does pyexiv2 really not support writing XMP tags to GIFs, even though GIFs support XMP? Or do I have some kind of error in my code?

I know pyexiv2 is techniacally deprecated in favor of gexiv2, but I get the same error with it. I also don't know how I would package gexiv2 for a windows installation. I'm developing on Linux, but I want my application to be cross platform. I also tried the python XMP toolkit, but that doesn't work on Windoes either.

If exiv2 really doesn't support GIFs, is there any python library that does, and works cross platform?