None of the tools below are GTA VI-specific, because none exist yet. What follows is the general-purpose toolkit that every recent GTA modding era — from the OpenIV years through FiveM’s asset pipeline — has ended up standardizing on, and that is worth being fluent in before there is a game-specific reason to be.
Blender, for anything 3D: vehicles, props, interiors, character work. Free, actively maintained, and the closest thing modding communities have to a shared language for meshes.
A proper image editor with layer and channel support — GIMP if you want free, Photoshop or Affinity Photo if you already have a license — for texture work, normal maps and UI art.
A hex editor and a general familiarity with binary file formats. Every past GTA title’s asset pipeline has eventually been reverse-engineered by people comfortable reading raw bytes before an official or community tool existed to do it for them.
Git, even for solo projects. The moment a mod has more than one contributor — or you just want to stop overwriting your own work — version control stops being optional.
And a scripting language: Lua has been the standard for FiveM-style server scripting, C# and C++ show up constantly in native modding tooling. Which one matters less than being comfortable enough in one to read someone else’s code.