head to head
Neither is simply harder - they are hard in different places. Desert Strike: Return to the Gulf demands more optimization; Grand Theft Auto V demands more reads.

Execution · Practiced Mastery
Helicopter handling under fire while routing fuel and ammo across fixed missions.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Desert Strike: Return to the Gulf leans Execution (Practiced Mastery); Grand Theft Auto V leans Execution (Practiced Mastery).
Desert Strike: Return to the Gulf leans a little harder on execution, though the gap is small.
Desert Strike: Return to the Gulf
Helicopter inertia and dodging fire demand real stick control
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans clearly harder on reads.
Desert Strike: Return to the Gulf
Defenses are fixed emplacements, nothing adapts
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Desert Strike: Return to the Gulf
Maps are unknown on first sortie only
Grand Theft Auto V
Some mission and lobby randomness.
Desert Strike: Return to the Gulf leans clearly harder on optimization.
Desert Strike: Return to the Gulf
Fuel, ammo and mission-order routing is the strategic core
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.