Whiterun city gate/door is missing in Skyrim (original 2011 version)

So I've been having a strange issue. I came back home one day, and decided to play Skyrim. Had some issues with Steam refusing to start; I got it working, then opened Skyrim and went to Whiterun only to find that there's no gate/door to enter the city. I can still go inside by interacting, but there's no texture, and just a black void instead, which I can walk into and fall out of the world.

I tried to reinstall the game, and verify file integrity (Steam version with all DLCs). This was a modded version, but I've completely deleted all the mods (and reinstalled the game as I mentioned). The issue happened like a switch, one day its fine, the next it's broken.

I can provide screenshots if needed.

The most probable cause is a rogue texture, I don't think it's a script mod as the model is still there (otherwise you couldn't activate it). So let's make sure there's no erroneous mod files left over (NOTE: only use this method if you don't have mods installed anymore, otherwise it will likely break your mods!); head into your Skyrim directory and open the data folder - if you see models or textures then delete them. Inside the data folder you should also only see .esp, .esm and .bsa files for Skyrim, Update and the DLC's (including high res texture pack). If there's any other .esp, .esm or .bsa files remove them too.

It's possible that a script mod has done something peculiar and replaced the door with a model without a texture, or some such action. In this case you should clean the save - the only problem in your case is that requires you to have the mods installed; see here for cleaning the save. You could try to skip the step where you uninstall the mods and see how that plays out. Alternatively as you have a list of the mods you used you could reinstall them and then follow the steps; I would focus on Skyrim HD, A Quality World Map, SMIM and Green Lods as they modify models and textures but the safest bet would be in install them all then follow the steps in the link I provided.

Edit:

Just to explain; even though you reinstalled the game the uninstallation that comes with that will have only removed the files that it knows about - the added mod files would have been left behind. Mod uninstallation isn't great at cleaning up mod files either.