Using following command to see what version of gtk my system is using: apt-cache policy libgtk2.0-0 libgtk-3-0
It says:
libgtk2.0-0:
Installed: 2.24.27-0ubuntu1
Candidate: 2.24.27-0ubuntu1
Version table:
*** 2.24.27-0ubuntu1 0
500 http://archive.ubuntu.com/ubuntu/ vivid/main amd64 Packages
100 /var/lib/dpkg/status
libgtk-3-0:
Installed: 3.14.13-0ubuntu1
Candidate: 3.14.13-0ubuntu1
Version table:
*** 3.14.13-0ubuntu1 0
500 http://archive.ubuntu.com/ubuntu/ vivid-updates/main amd64 Packages
100 /var/lib/dpkg/status
3.14.12-0ubuntu2 0
500 http://archive.ubuntu.com/ubuntu/ vivid/main amd64 Packages
It seems to be saying I've got two versions installed. Why so?
They are different ABI versions of GTK+.
As mentioned in comments, some things still require GTK+ 2.x, and haven't been ported to GTK+ 3.x. Likewise, you may have both Qt 4.x and 5.x installed, as different applications require different versions.