You can mod GTA 5 in two ways: With .ASI mods and with .DLL mods. Before installing any GTA 5 mods, you'll also need to download some extra add-ons like Script Hook V ...
Unreal Engine Plugin for adding Lua scripting to your projects. If you want modders to customize your game/project, or you need to allow game designers to script parts of the logic, or more generally ...