If you (and your team, if applicable) already know that set of languages, then there's no reason for you to learn Go.

Indeed, if you already know Java, the only reason to learn Go would be if you had a burning need to produce self-contained statically linked binaries. For example, at my company, we have a habit of producing command-line clients for our services, and Go is a good choice for that.

Go has been most useful for people who know a dynamic language like Ruby or Python, but who are encountering problems where they need more performance, better concurrency, or stronger static guarantees. Go improves over dynamic languages in those respects, and it's pretty easy to learn, so it's a great addition to their toolbox.