"Leave" is gadael, and I'd say it's not quite the same as "go"/mynd. Duolingo mostly likes really literal translations, but you never know. There's no need to worry about nitpicking though - the Beta release is all about constructive criticism!
I love reading the "nitpicking" and occasionally "contributing" - it is constructive and enjoyable. I'm a relative newbie on here. It's part of the course's attraction, keeping me working on my Welsh. Diolch :)