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. :)
These two languages accomplish different goals. Go is only 20% slower than C. If you want crazy speed and easy parallelism, then learn Go. However, readability was not a design goal. Also, it has no exceptions. It can be used for real-time products though ...