head to head
Warhammer 40,000: Space Marine 2 asks more than Grand Theft Auto V, and the gap is execution.

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

Execution · Practiced Mastery
Co-op-focused action sequel hurling Titus against Tyranid swarms with parry-heavy melee combat.
Grand Theft Auto V leans Execution (Practiced Mastery); Warhammer 40,000: Space Marine 2 leans Execution (Practiced Mastery).
Warhammer 40,000: Space Marine 2 leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Warhammer 40,000: Space Marine 2
Parry, dodge and gun-strike timing against swarms
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Warhammer 40,000: Space Marine 2
PvP Eternal War adds some human prediction
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto V
Some mission and lobby randomness.
Warhammer 40,000: Space Marine 2
Tyranid swarm spawns vary run to run
Warhammer 40,000: Space Marine 2 leans a little harder on optimization, though the gap is small.
Grand Theft Auto V
Heists, money loops, builds learnable.
Warhammer 40,000: Space Marine 2
Class builds and perk loadouts reward planning
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.