Those who have installed mods for GTA 5 may have used a tool named Script Hook V for the same at some point.

To install the contents of this downloaded folder into Grand Theft Auto 5 story mode, you must first install and set up Script Hook V and OpenIV. Both of these are essential if you want to install ...

Installation is fairly simple, you need the latest version of ScriptHookV.dll, Asi Loader (Bundled with Script Hook) and the mod files (.asi).