Hey, I don't want start a war, but I heard that GTK3 is more advanced, Windows fully support and more efficient, etc.
Is GTK3 really that advanced compared to QT? If so, KDE team will consider to use GTK and ditch QT?

Sorry, but no, this is nonsense. Also please do not cross-post this all over the forum, it will not make it better.

Also it is Qt, not QT which would be something completely different.

Before posting silly questions and rely on "I have heard" by unnamed sources you should have a look at Qt 4 and the soon to be release Qt 5. It is cross-platform since the start of Qt 4, which was about 7 years ago: http://en.wikipedia.org/wiki/Qt_(framework). You seem to ignore totally that KDE also runs on Windows since quite some time...

+1 Mamarok said it better than I would.

Here are some comparisons:
http://www.wikivs.com/wiki/GTK_vs_Qt
http://phil.freehackers.org/kde/cmp-toolkits.html

I usually hear the following reasons for using GTK+ instead of Qt:
1. Licensing issues.
2. GTK+ is written in C (while Qt is C++) and has more bindings.
3. GTK+ is shipped with more distros.

Point 1 is not very relevant anymore since Qt is also licensed under LGPL since 2009.

2. is mostly about preference, some stay away from Qt simply because they dislike C++. Personally I think an object-oriented language makes sense for a toolkit. In terms of features etc., I'm pretty sure Qt is considered more "advanced", although I don't have any references to back this up at the moment.
As far as I know GTK+ does have more bindings than Qt, so if you want to write your application in e.g. Vala, the former may be more attractive.

I don't really understand why people still argue about 3. - most distros come with package managers that will install all dependencies, and most people don't exactly lack hard drive space nowadays (as far as I know Qt4 is less than 100 MB). I've also seen people write that installing a Qt application will pull in the whole kdelibs, which is simply not true.
Some time ago I read that Ubuntu would ship Qt by default but I'm not sure what happened to that.

In any way, even if GTK+ were superior to Qt, I doubt there's enough manpower to port all KDE workspaces and applications to it, so this discussion is moot.

> so if you want to write your application in e.g. Vala

Vala is not really a fitting example since the language itself exists due to Gnome developers getting fed up by programming with a pseudo-object-oriented toolkit in a non object-oriented language

As for the bindings, Qt has them for all relevant ones (java, .net, python, ...), and then some (including even Haskell). GTK, since it is a C library doesn't really need bindings most of the time since C is /lingua-franca/ of most programming language's native interfaces.