Question: I was reading about Go, and I can see that it's very good and can be a language used by many developers in some months, but I want to decide a simple thing: **Learn Go or improve my Python or Ruby knowledge?**

Years developing with Python: 1  

Years developing with Ruby: 0.3

Answer: 
How long have you been working with Python?

If it were me, I'd do my best to maybe get a working knowledge of Go (basic syntax, some familiarity with unique language features), and continue with Python as I normally would.

Eventually you might come up with a small project that Go seems suited for (or you can come up with one now!) and really dive into the language that way.

There's no reason to limit yourself to just one. :)
Python is a available for most operating systems, it's generally accepted as a scriptin language, and it has matured to production quality.

Go is a research language that's only available in beta quality on Linux and OS X. Nowhere else. It's interesting from an intellectual point of view in that you can learn and apply a few concepts (typed channels combined with easy multitasking) that are otherwise difficult to use. 

As for Go, you might perhaps take a look at AT&T's "Plan 9" operating system first. It comes with a programming language called "Aleph" with also is C-based, has channels and multitasking. Looks to me like Go is a reimplementation of Aleph on more main-stream operating systems.