head to head
Neither is simply harder - they are hard in different places. Gran Turismo 3: A-Spec demands more execution; Grand Theft Auto V demands more reads.

Execution · Practiced Mastery
A PS2 driving simulator centred on mastering tracks and tuning cars through events.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Gran Turismo 3: A-Spec leans Execution (Practiced Mastery); Grand Theft Auto V leans Execution (Practiced Mastery).
Gran Turismo 3: A-Spec leans clearly harder on execution.
Gran Turismo 3: A-Spec
Lap precision against the clock is the core test
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans clearly harder on reads.
Gran Turismo 3: A-Spec
AI opponents never adapt
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Gran Turismo 3: A-Spec
Deterministic physics, minimal randomness
Grand Theft Auto V
Some mission and lobby randomness.
Gran Turismo 3: A-Spec leans clearly harder on optimization.
Gran Turismo 3: A-Spec
Optimal cars, tunes, and racing lines are findable
Grand Theft Auto V
Heists, money loops, builds learnable.
A matching score does not mean a matching test. Two games can both sit at 10 on execution and demand completely different things of you - which is why the reasoning above matters more than the number beside it.