head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is execution, and it is a single point.

Execution · Practiced Mastery
A modern Gears entry spanning campaign, Horde, Escape, and ranked versus.

Execution · Pure Mechanics
Trilogy capstone whose equal-start arena multiplayer defined a console generation.
Gears 5 leans Execution (Practiced Mastery); Halo 3 leans Execution (Pure Mechanics).
Halo 3 leans a little harder on execution, though the gap is small.
Gears 5
Movement tech and shotgun duels remain the core test
Halo 3
Precision aim and grenade-melee timing
Halo 3 leans a little harder on reads, though the gap is small.
Gears 5
Ranked versus rewards out-thinking human opponents
Halo 3
Equal-start fights reward predicting humans
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Gears 5
Escape and Horde inject some run variance
Halo 3
Equipment pickups and spawn variation
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Gears 5
Character ultimates and map control are learnable
Halo 3
Power-weapon timers and map control
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.