It contains a few directories which normally get unpacked into the Netscape plug-in SDK main directory. So, to unpack, first grab the Netscape SDK and put it on your disk somewhere. You will end up with a directory called 'gl-plugin'. In there, there are two Microsoft Visual Studio 6 workspaces in the directory 'Viewer' and 'Winproject'. The workspace in 'Winproject' builds the Netscape plug-in.
Time is the thing.
Free software is done in free time, so no other platforms have been done. There is one file 'windows.c' which contains the platform specific code. If you want to do an X11 port, then just create an X11.c, implement the functions in there and send it to everyone else to use - we're happy to place the binary here to download if some kind soul feels like doing that. Same reason as why only Windows - time. I tried, but it just took too long and I gave up.