There's a much simpler way to do this using only a single filter
Note I did this with a png but it should probably work for a gif as well
Go to Filters
Add a Scroll Filter
Enable Limit Width, Limit Height, and Loop
Set the Limit Width & Height to size you want
Don't worry that if the "limited size" is larger than the image itself, as long as Loop is enabled it should tile
I set mine to 1920 x 1080 to cover my whole display but yours my be different depending on your display size.
Also I realize this post is about 7 years old at this point but I was trying to find this information myself and couldn't find any solution besides the one above or some older videos on youtube that were saying things like "make the image bigger in your art program" or "copy paste the source a bunch of times"