Grand Theft Auto 5 Role Play or RP is a completely free-to-play multiplayer mode that can be added to the base game via mod clients like FiveM and RageMP. These mod clients host multiple RP servers that anyone can join. Once allowed in, a newcomer is required to create a custom character and its unique backstory.
Completing missions is the intended way to gain RP, as they're the bulk of GTA Online and contain all sorts of objectives and play styles. You can compile some of the missions that give out the most RP in a playlist, either by yourself or with a crew, to tackle them efficiently.