"high quality gif" is a misnomer. You have up to 256 colors per image/frame, there is nothing high quality about it.
The application does look pretty good and the sketch integration seems like it could also be interesting. I'm still not sure it's better than Gif Brewery especially since Gif Brewery is now free, but the site, and app look pretty nice.
Gif Brewery is great for turning videos into GIFs. Gifmock can do that, but can also help you create GIFs from your mockups, and that's what people tend to use the most ;) Regarding quality, you can create retina GIFs with Gifmock, and 256 colors is the limit.
Let me know if you have any questions.
256 color is a limitation of the GIF format: