head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Grounded 2 demands more optimization.

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

Optimization · Practiced Mastery
Backyard survival testing crafting optimisation with timing-based bug combat.
Grand Theft Auto V leans Execution (Practiced Mastery); Grounded 2 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.
Grounded 2
Dodging and perfect-blocking giant bugs takes real timing
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Grounded 2
Insect AI follows mappable behaviour patterns
Grounded 2 leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Grounded 2
World discovery and resource spawns hide some information
Grounded 2 leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Grounded 2
Base building, armor builds and tech routing drive progress
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.