Some "ultra low spec" mods replace high-res textures with 256x256 pixel versions, but even then, the base game engine, map geometry, and script files total over 8 GB.
depending on updates. Compressing nearly 100,000MB of high-quality textures, audio, and complex code into 100MB (a 99.9% reduction) is technologically unfeasible. Why "100MB GTA V" Downloads Are Dangerous gta v highly compressed 100mb