These are skeletal PK3's that, when placed in GZDoom's IWAD search path, will allow for selecting the relevant games from the IWAD selector. No content from these games has actually been placed into these files.
I highly recommend you do not install these unless you have the required files for it. Obviously I cannot include them here for copyright reasons. SIGIL is available for free here, with the optional Buckethead music addon available for purchase, and you can get NRFTL by purchasing the BFG edition here.
Not everyone is going to like the additional IWAD entries this creates. If you feel like your IWAD list is a bit cluttered, don't use these.
Also - ESTIERCOL made one for the "masterlevels.wad" distributed with Doom Classic Complete on PSN. Thank you! Here's the link: download/file.php?id=36979