While GTA Online more than satisfies players' needs for a massively multiplayer romp in Los Santos and Blaine County, there isn't any kind of LAN or Co-Op mode included in the game. Seeing as there are three protagonists, missing a feature like this is more than a little baffling.
While fan-made multiplayer mod projects for GTA 5 were relatively common some time ago, many sought to replace the official Online mode for their users - some even tried to turn a profit. They advertised themselves with "features" such as allowing mods while online and hosting your own servers. However, these modded services were unstable and had low populations.
They weren't particularly long-lived either. Some time ago Rockstar cracked down on all of these modded multiplayer servers since they directly conflicted with the EULA and TOS and were also leading to loss of revenue. Some operators understood and accepted this gracefully - others didn't.
However, Rockstar left the LAN and Co-Op mods alone. These were characterized by allowing a small number of players connect and play within the same instance of GTA 5's single player mode - therefore they didn't function as Online alternatives.
One of the most popular of these Co-Op mods was rather intuitively named GTACoOp. However, support and updates for it seemingly ceased a few months ago in January, though the mod still could use a bit of tweaking.
An industrious modder and fan of GTACoOp decided to unofficially "take over" development and released his own internet and LAN Co-Op mod for the game, which is largely based on the framework of its predecessor.
TinyMP allows several players to enter the same, otherwise regular, single player GTA 5 world. The main improvements over its ancestor is that TinyMP allows players to host their servers from within the game, with the aid of a rather simple menu.
The author, Soccermitchy, is planning on expanding functionality making the use of the mod much more convenient. Bug fixing and new features are also high on the priority list, depending on the popularity of the mod. While GTA Online offers a solid multiplayer experience, every now and then players might feel the urge to faff about the single player world with a few friends along.
Do you plan on playing GTA 5 co-op mods?