Step one is acquiring the images.
Easy: many webcam software have automatic picture scheduling. In Webcam Zone Trigger, you need to go to the Video Settings tab, then click on the Camera Timer button.
Images will be round up in the save folder, and from there you can just select everything and drop them in a photo editing software. I use Gimp, which is free. In Gimp, Drag & Dropping a bunch of pictures will automatically distribute them on separate layers. You can then crop and resize the whole thing, and save them to the .gif format. You may be prompted to either flatten the layers to a single image, or produce an animation using each layer as a frame, choose animation.
Simple eh? :-)