FiveM was the alternative multiplayer framework for GTA 5. It was unofficial, then tolerated, then in 2023 Rockstar acquired the company behind it. That changes how the GTA 6 question reads.
For the wider RP question, see the RP server expectations piece.
What FiveM was for GTA 5
A separate multiplayer framework that ran on top of GTA V's PC build but bypassed Rockstar's official multiplayer entirely. Servers ran their own gamemodes - RP, racing, custom missions - and shipped their own resources. By 2020 onward, FiveM concurrent users frequently exceeded GTA Online's.
FiveM was the home of the entire serious GTA RP scene - NoPixel and dozens of other servers. It was also the home of most serious map modding.
What changed in 2023
Rockstar / Take-Two acquired Cfx.re, the company behind FiveM. The acquisition framed FiveM as a legitimate part of the broader Rockstar ecosystem rather than as an unofficial alternative.
What this means for GTA 6 is uncertain. Rockstar could ship a FiveM-equivalent on day one. Rockstar could fold the framework into official multiplayer. Rockstar could leave it independent. None of this is decided publicly.
What's likely to happen
Speculative. The simplest read: Rockstar uses Cfx.re technology to support some level of community server framework for GTA 6, on PC, after the PC release. Whether that is exactly FiveM-shaped or something more integrated is unclear.
What's unlikely: GTA 6 launching with no community server framework at all. The demand is too large and the technology is now in-house. Some version of community servers seems likely. The shape is open. Once whatever framework launches stabilises, the discovery side - browser, search, categories - will likely live on dedicated multiplayer server discovery sites the way the GTA 5 scene did.