If I put on my marketing hat, the answer is that Go can be used for any project you want. There are libraries that will allow you to create Web apps, GUI applications, controllers for embedded systems, and much more.
If I take the marketing hat back off again, the more complicated answer is that go specializes in systems programming. So think of utilities that systems administrators might use to set up files, run network services, etc. That's the kind of thing it's easiest to program in Go. The prime example of this sort of thing is Docker, which is used to simulate an entire operating system for an app to be packaged within.
Go is also really good at concurrency - handling multiple tasks seemingly at the same time. That makes it a great platform for writing web apps and web services.