head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; The Legend of Zelda demands more optimization.

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

Optimization · Practiced Mastery
The original open-exploration adventure where hidden secrets and dungeon knowledge are everything.
Grand Theft Auto V leans Execution (Practiced Mastery); The Legend of Zelda 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.
The Legend of Zelda
Tight sword-poking against aggressive enemies
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
The Legend of Zelda
Enemies move on fixed or random patterns
The Legend of Zelda leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
The Legend of Zelda
Hidden caves and burn-every-bush secrets
The Legend of Zelda leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
The Legend of Zelda
Dungeon order, item routing, and secret knowledge
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.