Some helpful notes for myself and other people looking into modding Gran Turismo 6. This goes beyond Nenkai's Modding Hub and specializes in strictly GT6 files and data.
While GT6 uses many of the same types of file types and features and many of the same tables in its databases, the way it uses them can be very different than previous titles. Nenkai's Modding Hub mostly focuses on on GT4 specifications, which GT5, and to a lesser extent, GTPSP shares a lot in common with. GT6 is the weird middle child between GT4 and GTSport. It uses standard SQL database files instead of proprietary ones, plus it uses a knockoff of JavaScript for all its scripting, as opposed to whatever the previous titles used. GT Sport uses very similar formats to GT6, however I've not attempted GT Sport modding since I don't own a PS4. GT7 switched to using Swift for all its databases and scripting. These notes won't go over stuff already covered in GT Modding Hub unless there's something to add or it's radically different in GT6. I won't tell you how to install mods and whatnot or how to mod your PS3, that's on you to find and figure out. This is for the people who actually plan to get into the nitty gritty of GT6 modding!
Always make sure you've copied your PDIPFS folder from your GT6 install and not from sources online. If your PDIPFS source folder isn't around 20GB or so, you'll be missing a lot of files! If your GT6 install is disc based, you'll have GT.VOL, the disc file, and PDIPFS, the updates plus whatever mods you previously installed and are basing your new mod on. Digital GT6 installs will only have PDIPFS.
Either way, you'll want to copy GT.VOL and PDIPFS off your PS3 and to your PC (just use FTP, PS3 only supports FAT32 and the exFAT and NTFS mods don't really work too well when copying off the PS3). After that, you'll extract both into separate folders via GTToolsSharp.exe. You can extract them in whatever folder name you want, but make sure you always keep GT.VOL and PDIPFS in the GTToolsSharp folder. Never touch add or remove anything from the PDIPFS folder, as it's what GTToolsSharp references in order to pack your mod.
You can and probably should copy files from that folder into your mod if needed. Feel free to copy stuff from GT.VOL (if you have it) for stuff not included in PDIPFS (just be aware that the files in PDIPFS are newer, so if stuff copied from PDIPFS asks to overwrite files, let it do so). Just remember this rule: PDIPFS is untouchable, it's like a valued book, you can look and copy from it, but never write to it. You'll instead be writing your own PDIPFS in either its own folder or renamed as PDIPFSxxx (xxx being whatever you want to call it).
Your mod's PDIPFS will always have at least two folders, but it can have more. There will always be a 9 and K folder. The 9 folder is where your mod actually resides, K is basically the update header so the game knows what version/mod your mod is based on. Always install 9 first, then K. Forgetting K can cause the game to crash upon boot. This is what causes most GT6 crashes, but other things can cause that (as I'll eventually get to). In my experience, it's possible for GTToolsSharp to spit out more than just 9 and K folders for your mod. As far as I'm aware, there's no harm in not installing those files, but there's absolutely zero harm in installing them. If you install them, it's possible that reversing your mod back to the previous install would be trickier. I'd have to ask Nenkai or ddm about that one.
The DB0106.db file is where most GT modding occurs, GT6 is no different. Due to its size and complexity, it has its own section at https://github.com/SonicBoone56/GT6-Modding-Notes/blob/main/SpecDB.md.