head to head
Neither is simply harder - they are hard in different places. Doom: The Dark Ages demands more execution; Grand Theft Auto V demands more reads.

Execution · Practiced Mastery
A medieval-flavored Doom prequel built around shield parries and heavy combat.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Doom: The Dark Ages leans Execution (Practiced Mastery); Grand Theft Auto V leans Execution (Practiced Mastery).
Doom: The Dark Ages leans clearly harder on execution.
Doom: The Dark Ages
Parry timing and shield play demand precise inputs
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans clearly harder on reads.
Doom: The Dark Ages
Demon attack patterns are fixed and learnable
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.
Doom: The Dark Ages
Encounters fully scripted
Grand Theft Auto V
Some mission and lobby randomness.
Doom: The Dark Ages leans clearly harder on optimization.
Doom: The Dark Ages
Upgrade paths and combat-loop mastery solve fights
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.