GIF is a remarkably inefficient way to store animation. The format dates from 1987, compresses poorly, and supports only 256 colours per frame. A few seconds of high-quality animation can easily reach twenty megabytes as a GIF, which at Imgur's scale is an enormous bandwidth problem. Imgur solved it by converting every uploaded GIF to MP4 — typically around a tenth of the size, at visibly better quality — and inventing the "gifv" label for the page that plays it on loop.
The important consequence is that gifv is not a file format. It is a web page. Saving a gifv link with a naive tool gives you an HTML document rather than any media at all, and following it correctly gives you an MP4. Neither is a GIF, because there is no GIF anywhere on Imgur's servers to retrieve — the original was discarded after conversion.
If what you actually need is an animated GIF, for a chat application or a forum that will not accept video, it has to be rebuilt from the MP4. That conversion is genuinely lossy in a way worth understanding: reducing to 256 colours per frame introduces banding in gradients, and the resulting GIF is usually several times larger than the MP4 it came from. Both trade-offs are unavoidable properties of the format, not a limitation of the conversion.
One smaller Imgur quirk worth knowing: when an image has been deleted, Imgur serves a placeholder graphic rather than returning a not-found error. To an automated tool that looks like a successful download, which is how people end up with folders full of identical placeholder images. We detect the placeholder and tell you the image is gone instead.