head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Nation Red demands more execution.

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

Execution · Practiced Mastery
An arena twin-stick survival shooter about aim, movement and perk optimization.
Grand Theft Auto V leans Execution (Practiced Mastery); Nation Red leans Execution (Practiced Mastery).
Nation Red leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Nation Red
Twin-stick aiming and dodging endless zombies is twitch-heavy
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Nation Red
Zombies swarm mindlessly, nothing to predict
Nation Red leans clearly harder on variance.
Grand Theft Auto V
Some mission and lobby randomness.
Nation Red
Spawn rushes and perk drops add chaos
Nation Red leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Nation Red
Perk builds and positioning routes extend survival
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.