FiveM was GTA 5's alternative multiplayer modification framework. It started off unofficial, became tolerated, and then in 2023 Rockstar purchased the company that made it. That alters the outlook on the GTA 6 situation.
For the broader RP question, head to the RP server expectations piece.
What FiveM was for GTA 5
A multiplayer modification that ran on top of GTA V's PC version but ignored Rockstar's own multiplayer component altogether. Servers implemented their own gamemodes - RP, racing, custom missions, etc. - and provided their own resources. From 2020 onward, FiveM often surpassed GTA Online in concurrent users.
FiveM hosted the entirety of the serious GTA RP scene, including NoPixel and dozens of other servers. FiveM also hosted most serious map modding.
What changed in 2023
Rockstar / Take-Two have bought FiveM's parent company, Cfx.re. FiveM is now considered a legitimate part of the larger Rockstar ecosystem, rather than an unofficial alternative.
How this applies to GTA 6 is unclear. Rockstar could launch a FiveM equivalent on day 1. Rockstar could integrate the framework into official multiplayer. Rockstar could keep it entirely separate. No word on any of that publicly yet.
What's likely to happen
Speculation. The easiest explanation: Rockstar implements Cfx.re technology in order to provide some manner of community server support for GTA 6 on PC after the launch of the game's PC version. Whether it's FiveM-shaped or something more integrated remains to be seen.
Unlikely: GTA 6 launching with absolutely zero framework for community servers. There's too big a demand and they own the tech outright now. Some flavour of community servers seems likely. How it looks is still wide open. Once whatever framework they launch stabilises, the discovery side (browser, search, categories, etc.) will likely live on dedicated multiplayer server discovery sites, the way the GTA 5 scene did.