This suggestion will tell you which minor version of 2.0 is installed. Different major versions will have different package names because they can co-exist on the system (in order to support applications built with older versions).
Even for development files, which normally would only let you have one version on the system, you can have a version of gtk 1.x and a version of gtk 2.0 on the same system (the include files are in directories called gtk-1.2 or gtk-2.0).
So in short there isn’t a simple answer to “what version of GTK is on the system”. But…
Try something like:
dpkg -l libgtk* | grep -e '^i' | grep -e 'libgtk-*[0-9]'
to list all the libgtk packages, including -dev ones, that are on your system. dpkg -l will list all the packages that dpkg knows about, including ones that aren’t currently installed, so I’ve used grep to list only ones that are installed (line starts with i).
Alternatively, and probably better if it’s the version of the headers etc that you’re interested in, use pkg-config:
pkg-config --modversion gtk+
will tell you what version of GTK 1.x development files are installed, and
pkg-config --modversion gtk+-2.0
will tell you what version of GTK 2.0. The old 1.x version also has its own gtk-config program that does the same thing. Similarly, for GTK+ 3:
pkg-config --modversion gtk+-3.0