Two main problems with gifs are that they use lossy compression, making them less desirable than pngs, and there's no simple way that I know of to read animated gifs into the program. As far as I know all animations in terrara consists of multiple frames in a single png file.
For that you can use the stack parameter in SetResult. It's 1 by default if you don't provide a value. For example, the example mod lets you craft 15 life crystals from one dirt block.