No, the application can't be used to rip the images of GIF format. After the Tumblr address is inserted and the project is started, the pages will be parsed, and then the images will be downloaded to to the folder on your computer specified by you. I haven't see any GIF support for the application, unless the images on the page are in GIF format.
However, you can convert the downloaded images to GIF with an image editor. You can try XnView application.
