Many thanks to the Code Alliance for the JK Specs hosted at https://www.massassi.net/jkspecs/ Without their work of documenting JK file formats, this viewer would not have been possible.
After you have checked out the repository, copy the Jedi Knight game files with the extension GOB
into the root directory of the repository: JK1.GOB, JK1CTF.GOB, JK1MP.GOB, Res1hi.gob, Res2.gob.
For Mysteries of the Sith the extension is goo and the game files named JKMRES.GOO and JKMsndLO.goo;
note that these need to be added in addition to the JK files.
If you don't own the game, you can download the demo from https://archive.org/details/StarWarsJediKnightDarkForcesII_1020 (it comes as an installer for Windows).
Then, initialize a local Python environment using pipenv install. Note that pipenv needs to be
available on your system. In case you run into any problems with the install command, try
pipenv update to update dependencies; they hopefully then install successfully. Next, switch your
shell into the environment using pipenv shell.
Before starting the server that allows you to view JK maps in your browser, you need to configure
it: Make a copy of config.py-default and name it config.py. Comments in the file explain the
values you can set there.
Finally, start the server using the script start.sh. Then navigate using a web browser to, for
example, http://127.0.0.1:5000/level/?url=https://www.massassi.net/media/levels/files/jkmp/baronnight.zip.
The url in the address is expected to be a ZIP archive of a map as hosted on the Massassi Temple.
See https://www.massassi.net/levels/ for all the hosted goodness there.
Note that if you own the game, you can add &ownsgame=1to the URL to see textures from the game files
in high resolution.