The scope and scale of the next GTA games has nothing to do with when they've started development. If GTA V at PS3-level fidelity took 5-6 years to make, then 10-12 years for a GTA VI at PS5-level fidelity seems logical.
If there's any truth to this rumour of a 2025 target release date, you can be almost 100% confident they're well into pre-production and prototyping at the very least.
To think they haven't started development at all is pretty silly, imho. What else can this multi-studio developer have been working on since shipping RDR2?