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

Optimization · Practiced Mastery
Co-op first-person zombie RPG about improvised weapons on a resort island.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Dead Island leans Optimization (Practiced Mastery); Grand Theft Auto V leans Execution (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Dead Island
first-person melee timing and stamina
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans clearly harder on reads.
Dead Island
zombies are scripted swarms
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Dead Island leans a little harder on variance, though the gap is small.
Dead Island
weapon drops and spawns add noise
Grand Theft Auto V
Some mission and lobby randomness.
Dead Island leans clearly harder on optimization.
Dead Island
weapon mods and skill builds carry late game
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.