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 ...

While Rockstar Games allows GTA 5 PC players to use mods in the offline version of the game, it also keeps updating the Rockstar Games Launcher which is directly connected to the game’s execution.