I have learned in my University time Pascal and C and RedHat Linux/Unix . 
To get quickly one job, i started learning Microsoft Visual Basic 6.0 for speed in development etc. In that time, with C its like more time consuming and i was not confident to use it for job purpose, where most of the companies demand fast/rapid development.
After that i had problems with my companies because they want web applications, then i started using PHP which is also great, because customers demand web projects and they expect Google like applications in short time frame, which is doable because PHP gives that speed and its huge community.
To explain my need for Go-lang is following:

PHP the syntax is friendly compared to C/Pascal. 
I was very happy to learn Python, but its syntax is very much different then C. 
Which just  not gonna work with me to accept and really learn it better and better. 
I have tried to learn Ruby, at-least so that i can have the knowledge of Python 
like syntax, but i really skipped Ruby because of 2x time slower then PHP

Therefore, 
Is Go-lang is the perfect choice for SPEED vs PHP vs Ruby, for Web development + Gtk?
Alas, I'd love to have 1 asset that I could use for all conditions but it's just not available in the world of computing. You're going to have to learn 2 or more.
PHP is very widely used, so you might as well stick with it. If you can create decent webapps using it, go for it. I would suggest learning C/C++ too so you can write any high-performance modules using that and call them from your PHP code. That's probably the best of all worlds for your webapps.
If you wanted to write for desktops, I think you'll be best off learning C++ with Qt (and look at Wt) (as it appears you're a Linux dev), or C#/VB.NET for Windows. 
For mobiles, learn C/C++ as you can write apps in that no matter which platform even if you have to put up with some platform-dependant extensions - you either have to learn Java for Android, Objective-C for iOS, or (well we're not quite sure what MS has planned for Windows Phone 8, but I hear they like native code again, that means C++/CX). You can see where I'm going with this!
so anyway, if you're happy with PHP then keep with it. There is a ton of code out there that runs PHP so it's not like you're working with some bleeding-edge or hardly-used obscure language.
Stick with PHP or learn Go-lang?