head to head
Gran Turismo 5 Prologue asks more than Grand Theft Auto V, and the gap is execution.

Execution · Practiced Mastery
Shaving lap time through precise car control and solvable line and tuning choices.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Gran Turismo 5 Prologue leans Execution (Practiced Mastery); Grand Theft Auto V leans Execution (Practiced Mastery).
Gran Turismo 5 Prologue leans clearly harder on execution.
Gran Turismo 5 Prologue
Braking points and throttle control decide every lap time
Grand Theft Auto V
Driving and shooting demand decent inputs.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Gran Turismo 5 Prologue
Online racecraft adds light wheel-to-wheel prediction
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 5 Prologue
Tracks and physics are fixed and fully knowable
Grand Theft Auto V
Some mission and lobby randomness.
Gran Turismo 5 Prologue leans clearly harder on optimization.
Gran Turismo 5 Prologue
Tuning setups and racing lines are an engineering problem
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.