Is PHP-GTK widely used for desktop application? Is it recommended to use? Is there a big company who uses PHP GTK?
No, it is not. Quite the opposite, it's practically a dead project. There was almost no activity for 3½ years now. It had never really got any traction, as PHP is very much web oriented, and there was not much interest in GUI library for it.
I've only seen it used in academic settings as an exercise in 'what not to do'.
With regard to scripting language GUIs the industry appears to be heading pretty heavily towards Python and it's subsequent libraries like PyQT and PyGTK. They're much easier to use, much better supported and much more proliferant than PHP.