If the code is compiled, you'd have to pull a few more tricks. But to be fair that is also an issue even if your code is not source available - people can (and do) decompile closed-source programs and hack on them. My personal favourite of that is https://multitheftauto.com/ where the community reverse engineered GTA San Andreas to add multiplayer. Source: about 1 year ago

I'm well into the nostalgia of games from my youth at the moment, so I've been playing Multi Theft Auto: San Andreas - the modded GTA SA with online multiplayer (https://multitheftauto.com/). Source: over 2 years ago

For context, one of these is FiveM[0], which is very popular (and if anything, a lot better than the official Online in many ways.) [0]: https://fivem.net/. - Source: Hacker News / 6 months ago

Alternatively, you could consider installing FiveM now that you’re on PC for GTA V. It’s a third-party multiplayer client that allows you to join all sorts of different servers that range from RP, to Freeroam, Deathmatch, etc. It’s pretty impressive in the fact that it allows for server hosts to implement literally anything, including mods for cars, graphics, maps, etc. Look into this more here: https://fivem.net/. Source: 12 months ago

It might not be official, but I'm pretty sure FiveM might have some servers with PVP disabled, or something like that. You can search for servers on FiveM here. Source: about 1 year ago

Https://altv.mp and fivem.net join a server, or build your private one for yourself. Source: over 1 year ago

I've seen some servers for it on youtube creating full Zombie Survival modes. Some servers are RP, some are kinda like GTA Online Clones, others are something completely different. https://fivem.net/ and /r/FiveM. Source: over 1 year ago