head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Psychonauts 2 demands more optimization.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.

Optimization · Practiced Mastery
A forgiving platformer testing light execution and exploration routing through fixed level designs.
Grand Theft Auto V leans Execution (Practiced Mastery); Psychonauts 2 leans Optimization (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Psychonauts 2
Moderate platforming and combat timing, forgiving by design
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Psychonauts 2
Solo adventure with no adaptive opponent anywhere
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Psychonauts 2
Level layouts and collectibles are fixed once seen
Psychonauts 2 leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Psychonauts 2
Routing collectibles and matching psychic powers to encounters
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.