head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Oddworld: Abe's Oddysee demands more optimization.

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

Optimization · Practiced Mastery
A cinematic puzzle platformer about sneaking a hapless Mudokon past guards while rescuing his coworkers.
Grand Theft Auto V leans Execution (Practiced Mastery); Oddworld: Abe's Oddysee 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.
Oddworld: Abe's Oddysee
Precise platforming and stealth timing kill instantly
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Oddworld: Abe's Oddysee
Sligs patrol on fixed scripts
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Oddworld: Abe's Oddysee
Deterministic screens, no randomness
Oddworld: Abe's Oddysee leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Oddworld: Abe's Oddysee
Mudokon rescue routes and puzzle solutions are solvable
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.