There is for example the game GTA SA I understand third party developers have made the client
Not quite. Any multiplayer game consists of two parts - client and server, and they don't work without each other (there are serverless games, but in such cases usually one client becomes the server, and it works in Doom/Quake/UT). In GTA the client and the server did Rockstar Games and not by individual developers. And even more, Rockstar Games have tried to do so in order to hide the communication Protocol - to the hackers and crackers do not crack the game by getting an advantage in the game non-gaming ways. "To make a server GTA", which earns the client the GTA, you can't, at least, the price of such development will be comparable to the price of the work on the GTA (which cost tens of millions of dollars).
What can a third party developer, to do fashion for ready server, using the tools that the designer suggested, or trying to "Rob" interaction Protocol. Mods can be server and client, and only if you have access to the computer running the server and the client (the client you always have, as do most mods).
For example to make a minecraft server is not a problem.
Minecraft is much simpler than GTA. Though, because minecraft was made by one person, and GTA made dozens of programmers. And here for minecraft, you can make your
server,not fashion (although fashion to it too). As an example, an excellent article
Downloaded, set up, launched
"I downloaded, configured, launched" is not "development servers", it took what was already developed by someone.
P. S. All written IMHO. I'm not a fan of GTA and don't know the market of unofficial servers GTA, only know that such is (sort of). But personally, I think that hardly anyone will be able to write
the server, if not or its source (to steal), or the developers will make tools to develop mods.