system
A chat on subreddit /r/pcmasterrace
human
You can trick the launcher.  It does look for mods automatically and I don't think you can even go online if you have them installed but like all things... it can be cracked.
gpt
The scripts are injected into the client. Rockstar has pisspoor security for the game as server game scripts are executed by the clients. Hell, every session has a player who functions as the script host.