head to head
Neither is simply harder - they are hard in different places. Call of Cthulhu: Dark Corners of the Earth demands more optimization; Grand Theft Auto V demands more reads.

Optimization · Practiced Mastery
Lovecraftian first-person horror where survival means learning Innsmouth's scripted terrors.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Call of Cthulhu: Dark Corners of the Earth leans Optimization (Practiced Mastery); Grand Theft Auto V leans Execution (Practiced Mastery).
Grand Theft Auto V leans a little harder on execution, though the gap is small.
Call of Cthulhu: Dark Corners of the Earth
Tense chase and shooting sequences need steady hands
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans clearly harder on reads.
Call of Cthulhu: Dark Corners of the Earth
Cultists follow scripts, never adapt
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Call of Cthulhu: Dark Corners of the Earth leans a little harder on variance, though the gap is small.
Call of Cthulhu: Dark Corners of the Earth
First-run dread from unknown scripted events
Grand Theft Auto V
Some mission and lobby randomness.
Call of Cthulhu: Dark Corners of the Earth leans clearly harder on optimization.
Call of Cthulhu: Dark Corners of the Earth
Learning routes, hiding spots, and sequences solves it
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.