Those Gifyyy URLs are pretty obvious. /img123 can only suggest that there's an /img122 and an /img/124, right? Well, who needs security by obscurity? It's not clear whether all GIFs or just those that are sent can be downloaded.
Since Gifyyy uses MeteorJS to fetch image URLs, we use PhantomJS to render the page and scoop the URL, which is slow.
Fortunately, this is easily parallelizable! Assuming GNU Parallel:
seq 1 1000 | parallel -j8 python --dest gifs/ --index {} <YourGalleryName>