There are lots of image editors that can easily join images of many types.
IrfanView is one of them (see command: Image>Create Panorama image). The problem you have is if you want animated GIFs to maintain their animation after the joining.
I don't know if any common editor can do that, I know IrfanView can't. I don't hold much hope for you finding any that can, because animated GIFs have a complexity of frame (or image slice - since not all animated GIFs use full frame updates) rate, the number of images per each GIF, and even the color pallets.
Good luck with that.