Are you saying that a jpeg thumbnail is being made of a gif image, or that you want to create a gif thumbnail from a jpeg image?

gif files are usually used for animations now, they don't really serve much purpose in other uses, where better file types such as jpeg or png (where transparency is required) are preferred.

For this reason, editing and thumbnailing of gif images is not available. Unless you really need to use a gif for some special reason, you should consider another image type, like png.