The GK3 Scene Viewer lets you view scenes (the rooms and whatnot) from GK3. It is capable of reading directly from .brn files, so you don't have to do any manual extracting. So easy a caveperson could do it (hopefully).
You can download the viewer via Sourceforge here. The latest version is 0.3.0.
If you want you can get the latest source from Subversion and build the viewer yourself. You can check it out of SVN from https://gk3tools.svn.sourceforge.net/svnroot/gk3tools/trunk/levelviewer, or you can view the source at http://gk3tools.svn.sourceforge.net/viewvc/gk3tools/trunk/levelviewer/.
The code is written in C# and .NET 2.0 (though it will build and run on Linux with Mono), and requires Tao 2.0.