Grand Theft Auto V [official site] modding hasn't been given any support by Rockstar, but this is PC-land and PC players will not be restrained. If you're interested in learning how you install mods for the game, I'll briefly lay out some basic instructions below. It's extremely simple, but it requires a couple of pre-installed files before most mods will work, and it can be intimidating if you're new to modding and the instructions are never all in one place.
GTA 5 Mod install guide
Original words by Graham Smith, updates by Dave Irwin
For some, the only mod they'll ever need is one that's already present in the game. I am of course talking about the various cheat codes that are present in the game. Check out our GTA 5 cheats guide for every cheat code present in the game, including those that spawn vehicles, give infinite ammunition, and lower your wanted level.
Before getting started, there are some notes about modding GTA 5 that you should definitely be aware of. Some of these things will only work in singleplayer, but some scripts out there will make changes to GTA Online. Be careful with this. GTA Online is dependent to some extent upon an in-game economy and if you start futzing around with online cheats - even if it's just making it snow - it's possible Rockstar won't take kindly to it.
That said, there's nothing you're doing here that can harm your game, your saves, or that can't be simply undone. To do this, right-click Grand Theft Auto V within your Steam Library, and go to Properties > Local Files > Verify Integrity of Game Cache. This will wipe away your changes and re-download any modified files.
Let's get started. You need to download just two things, Script Hook V and the LUA Plugin, and it won't take you more than three minutes to get it all working.
Script Hook V
Script Hook V has a descriptive name: it's a library of files created by Alexander Blade that allow enterprising modders to write scripts that then hook into Grand Theft Auto's singleplayer mode. This is the basic tool that makes modding possible. Download Script Hook V from here.
The package comes with both the most recent Asi Loader and the latest version of the Native Trainer. The Asi Loader is essential, the Native Trainer is not but does give you ready access to various cheats in the game, including teleportation, fast run and super jump.
How to install Script Hook V:
1) From within the \bin folder of the .zip you just downloaded, copy ScriptHookV.dll to GTAV's main folder. This is where GTA5.exe is located and, if you've got a Steam install, will be something like \SteamApps\common\Grand Theft Auto V\.
2. For the Asi Loader, you do the same thing but with dsound.dll from the same place.
3. If you want the optional native trainer, do the same thing but with the NativeTrainer.asi file. You probably want it because it lets you do this.
GTAV LUA Plugin
Now that you've got Script Hook V installed, there are some other basic mods that make other modding easier. The first is the LUA Plugin by headscript, which lets people write LUA scripts that work with the game. LUA is a language that might be familiar to you, at least in name, if you've ever futzed around with something like Garry's Mod; in short, it lets people make cool things. You can download the GTAV LUA Plugin from here.
How to install GTAV LUA Plugin:
1. Within the .zip you just downloaded, you'll find a folder called 'scripts' and a LUA.asi file. You need both.
2. Plop them in the same folder as before: \SteamApps\common\Grand Theft Auto V\.
3. Within the 'scripts' folder you just created, you'll see an 'addins' folder. This is where you're going to drop any LUA files you download in future. It'll be empty just now though.
How to install a mod in GTA 5
Now that you've installed the tools, let's put something in it. You've probably got something already in mind, but as an example, I've chosen something simple and you can download sakratt's Ragdoll/Limp On Demand script from here.
How to install GTAV ragdoll mod:
1. Inside the file you just downloaded, you will find a ragdoll-on-demand.lua file.
2. Drop it in the \SteamApps\common\Grand Theft Auto V\scripts\addins folder as mentioned above.
3. That is legits all. Press 'u' to perform the action in game and see the included readme.pdf for instructions on how to edit it so it's a toggle on/off button or a different key.
Tada! You can now mod Grand Theft Auto V. Now that you know, you can import lots of different scripts into the game, such as the aforementioned Ragdoll mod, or something more majestic like dropping blue whales into the world of Los Santos. You can take a gander at these here GTAV scripts.