> GIFs [are] fair use

Are they, though? Fair use only covers use for the purpose of criticism and parody (as in, making fun of the original work, not just making up your own lyrics to a popular tune).

> not using YouTube api

Even if you're not using their API, it still counts as a ToS violation. But since downloading YouTube videos is the main feature of your tool, there's not much you can do about it.