Why does Unity depend on Gnome and to what point is the dependency required?
Unity is not an entire graphical desktop. It is only one small part of it - the desktop shell. It is one of the most visible parts of an operating system, particularly when you are interacting with the controls it provides or launching applications.
In order to use a graphical desktop you need to have a solid suite of software which comprises everything from a window manager and display manager, to widget toolkits, to file managers, and countless GUI applets for configuring and controlling everything from networking to which application should open which types of files. You also need a suite of actual applications such as browsers, picture viewers, video players, and more.
Unity could have been written as a shell for any existing desktop suite. Gnome was chosen because it is relatively full-featured and had already been the default desktop environment for Ubuntu for many years prior. Thus, with the exception of what Unity provides (which has a very visible immediate difference), the vast majority of your graphical desktop works the same way you were used to from previous versions of Ubuntu, if you stayed with the default Gnome.
Unity is actually implemented as an extension to Compiz, which is not part of Gnome at all. Compiz is a 3D-accelerated window manager that is designed as an alternative to Gnome 2's metacity and Gnome 3's mutter, though it can be made to work with KDE as well (even though KDE now includes Compiz-like functionality in its own window manager). Ubuntu have chosen to work Unity and Compiz into the Gnome desktop environment, rather than KDE, for the reasons stated above. Some development effort would be required to make it work in KDE, even though Compiz itself can be made to work with KDE. The version of Compiz that is installed by Ubuntu makes use of a number of other Gnome-specific extensions in additional to Unity.
Why is a specific version of Gnome used for a specific Version of Ubuntu
This is just how most Linux distributions work - for a given release of the operating system, all their core software tends to stay at the same version throughout the life of that release, but a new release of the operating system will typically get newer versions of software.
The version of Gnome that ends up in each Ubuntu release will usually just be the latest version of Gnome that is "ready" (no major problems) at the time that Ubuntu is getting ready for release, given enough time for testing beforehand.
What changes is Unity doing in regards of being less dependent of Gnome
There's no reason to make Unity less dependent on Gnome, so I don't think there's anybody seriously working on that. If Ubuntu needed to drop Gnome for any reason (which I don't see happening) they'd probably also switch away from Unity, too.
Has the dependency on Gnome created limitations with Unity?
I'm sure that it has influenced Unity's technical decisions along the way, but it will also have granted Unity great freedoms as well, because it is a mature and full-featured desktop environment. Ubuntu are in no position, and would have no desire, to replace Gnome by building a new desktop environment from scratch.