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

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

Execution · Practiced Mastery
Arcade run-and-gun reflexes over memorised enemy placements.
Grand Theft Auto V leans Execution (Practiced Mastery); Top Gunner leans Execution (Practiced Mastery).
Top Gunner leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Top Gunner
Dodging bullet streams in the jeep is pure arcade reflex
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Top Gunner
Enemy emplacements fire 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.
Top Gunner
Layouts never change between credits
Top Gunner leans a little harder on optimization, though the gap is small.
Grand Theft Auto V
Heists, money loops, builds learnable.
Top Gunner
Memorising spawn points and POW routes maximises score runs
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.