I'm drag/dropping the doom_complete.pk3 directly onto the GZDoom executable, no other WADs or PK3s in the directory with it. Nothing in the INI autoload list. Not loading any other files.
I made a few different versions of doom_complete.pk3 to test if it was a problem with it not doing it right the first time (also to have the different orders for master levels and whether it has the midi or real music soundtrack for Sigil), but that hasn't fixed it.
doom_complete.pk3 is an IWAD (an IPK3, functionally the same thing - a primary resource file, same as DOOM2.WAD) so you don't drag it onto GZDoom.exe like you would an addon wad (PWAD). When GZDoom pops up with a window asking which IWAD to use, select doom_complete.pk3 - that is now your running IWAD, and you can access all the episodes etc. The reason GZDoom gave you that script error is because you were effectively loading doom_complete.pk3 twice.
I mean, I drag/drop doom.wad, doom2.wad, tnt.wad, plutonia.wad, chex3.wad, freedoom1.wad, freedoom2.wad, hacx.wad, heretic.wad, hexen.wad, hexdd.wad, and strife1.wad onto the GZDoom exe without issue. But I do notice that I get a similar error if I try it with hxdd,wad, and that it works right if I just have the file in the GZDoom directory (or the defined iwad directory) and choose it upon launch instead of trying to drag/drop it from another folder.
I mean, I drag/drop doom.wad, doom2.wad, tnt.wad, plutonia.wad, chex3.wad, freedoom1.wad, freedoom2.wad, hacx.wad, heretic.wad, hexen.wad, hexdd.wad, and strife1.wad onto the GZDoom exe without issue
The reason those work is because those are all vanilla Doom data and don't redefine any of GZDoom's cvars (console / user config values). The error you're getting is GZDoom saying that you're trying to define the same cvar twice, which you'd definitely get if you're loading doom_complete.pk3 as both an IWAD (as defined in your gzdoom.ini, which gets set when you pick an IWAD from the pre-launch window) and as a PWAD (by dragging it onto the EXE, which GZDoom always interprets as "load this WAD as an addon").
