What “Mi gone” means
Mi gone literally means "I'm gone" and is a very common way to say you're leaving. People say it as they head out, before they've actually gone.
Examples
Mi gone.
I'm leaving now.
Mi gone, inna di morrows!
I'm off, see you tomorrow!
How to use it
Casual and quick. On its own it can sound abrupt, so it's often followed by a softer goodbye like "likkle more" or "walk good".