These Google trends are only part of the picture.
Once you have found what you are looking for (e.g. Qt or Gtk+ documentation), you are likely to bookmark it and go directly to it, so your search result will only apply once even if you visit the Gtk+ docs a lot. Also, if you have downloaded and installed the Gtk+/Qt/KDE documentation, you don't need to search Google to find it!
Q: What are the Debian, et. al. popularity contest stats for the Gtk+, Qt and KDE libraries? What about the dev packages? Documentation packages? Applications making use of these libraries?
A user is not likely to search for Gtk+ or Qt (and maybe not even KDE or GNOME). They are more likely to search for things like "Amarok", "Natilus" or even more likely things like "PDF viewer for Linux". Even then, if they are looking for a specific application they are likely to go through something like the Ubuntu Software Center, or follow the instructions to install a cool new app from a blog/review site like http://www.webupd8.org./
A developer is also not likely to perform a generic search for the UI framework or desktop environment either. They will either search for a specific query mentioning APIs or packages such as "How do I set the value of a GtkProgressBar?", "gtk_label_set_text HTML styling" or "libqt5-gui". Even then, they are likely to try sites like stackoverflow first. They are also likely to ask questions on the relevant forums, IRC channels, etc.
Q: What are the search/question trends for gtk+/gnome and qt/kde tagged questions on stackoverflow?
Q: What are the visitor stats like and comment counts like on the various Qt/KDE/Gtk+/GNOME pages, blogs and wikis (e.g. Allen Day's GNOME blog).
Q: What are the trends for the number of commits and developers to the Gtk+/GNOME/KDE/Qt source code repositories?