(I don't know whether you have tried the following suggestions, as you haven't responded to the comment, but I thought I would mention the following ways, which are about the only options available.)

General Introduction

Without using compiz or some special compositor plugin, what you can do on Xubuntu is use xwinwrap; even though the program hasn't been updated officially for a while, there is an improved version available here with many comments posted by users.

However, you may want to use the excellent gwinwrap, which is a front-end to xwinwrap. You must install it by following the instructions from the official site; it is, however, very quick and easy. You also need mplayer and xscreensaver installed in addition to xwinwrap. Even though the documentation keeps mentioning screensavers, it does work for the desktop as well, as this useful youtube video demonstrates, and this is one way to make a desktop 'active'.

So, you could either use xwinwrap as it is and follow the tutorial here so that you can display your gif, or it may be easier to use gwinwrap. You could simply convert your gif to a video file and select it in gwinwrap and it would loop as the desktop background. Other options such as the size of the video could be passed to mplayer in the gwinwrap options.

If you run gwinwrap from terminal and select various settings and then apply them, you can see the xwinwrap command lines being used. As an example, gwinwrap executes an xwinwrap command (in conjunction with mplayer) such as this:

nice -n 15 xwinwrap -s -st -sp -b -ni -fs -nf -ov -o 1.000000 -- mplayer /home/mike/jazz.mp4 -quiet -noconsolecontrols -loop 0 -wid WID 

(To kill any xwinwrap actions which have gone awry, open a terminal and run killall xwinwrap.)

Specific instructions

1) Choose your animated gif from your collections and convert it to a video format such as mp4 or avi; you can do this directly with ffmpeg, as noted in the suggestions from this superuser question. If the quality isn't what you expected, try the other alternatives.

2) Start gwinwrap, click 'new' on the first screen, and then on the second, add your video of your converted gif, specify any settings and click apply (see screenshot below).

Note: you usually have to have window compositing enabled in XFCE settings > window manager tweaks. As noted in some of the comments for xwinwrap, sometimes the video may overlay other windows depending on your graphics card. It should not happen, however, and using xwinwrap/gwinwrap is about the best way to achieve what you want.