If you're using IM (v6.x anyway; haven't tested it with older versions) animated GIFs can be completely resized. I have vBulletin (3.5) set to use IM now, and a user just attached an animated GIF that was converted to a Thumbnail, and the thumbnail shows the complete animation.
(You have to register to view attachments, but here's the link just in case...)
http://www.fireblades.org/forums/showthread.php?t=38790
Haven't tested it in Gallery, but it's also using IM, so I would assume it would work the same.