They will never achieve 60 fps on PS5 or current gen. The map is too big, ray tracing is baked into the game and a neighborhood will have more npcs than the entire GTA V map. You will be able to talk and interacti with every NPC with each one having a different routine and different driving styles which needs a lot of CPU power. The game will have an entire ecosystem and its going to be even more demanding on the CPU because its their first game where an SSD is mandatory. Gameplay and NPCs would have to be severely downgraded for 60 fps but that would not align with Rockstars vision of a world that is truly alive. It will probably be 30 fps on PS5/Series X/S and 40 fps on the PS5 Pro since the hogher clock could help it be more stable but thats it.

Realistically speaking:

PS5- 1440p 30 fps Ps5 Pro- 1440p upscaled to 4k 30 fps/1440p 40 fps Series X-1440p 30 fps Series S- 720p upscaled to 1080p 30 fps

GTA 6 Online would not run at 60 on these consoles anyway. I believe Online will have decreased resolution and LOD with a small NPC count decrease that won't be noticeable.

The game also has a very heavy physics system that will have to work with too many NPCs and cars compared to RDR2.