what the right thing to google is
Not anything including "raspbian" or "raspberry pi". Raspbian is just a tweaked version of Debian, which is a standard GNU/Linux distro. If you search for things like this in a Pi specific way, you will tend to get pi specific answers (e.g., your tutorial involving the GPIOs) or stuff that is a regurgitation of existing linux oriented documentation. Commonly this is of lower quality than the original.
The GUI stack on linux, in contrast to MS Windows (or OSX), is heterogenous, meaning, it is not homogeneous, monolithic, or uniform across systems. The default stack used on Raspbian is not unique to it or Debian. OpenGL is cross-platform, so it is not even unique to linux. However, with regard to creating GUI windows, you want to work with a widget toolkit; the predominant (but not only) ones are Gtk+ and Qt. These are both cross-platform as well, although the former might be considered closest to a "native" GNU/Linux GUI API. The C++ version is gtkmm. I believe both these things should have some OGL integration although it has been a long time since I looked at them; in any case they both have extensive primary documentation.
I recommend you avoid anything which refers to GLUT as it exploits depreciated aspects of the OGL API. Once upon-a-time SDL was widely used to structure OGL applications but you may want to stick with something more minimal, again via the widget library if possible.
You can read more about what I mean by "heterogeneous stack" here. In relation to that, the default desktop environment used on Raspbian is a slightly customized LXDE, which uses openbox as the window manager. However, this is tangential or irrelevant with regard to developing GUI applications, which are creating using a widget library as mentioned above. DE's tend to use one or another for their own integrated applications (such as the file browser); LXDE is Gtk+ based, however, this does not mean you can only run Gtk based apps.