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
Multiplayer-focused Halo with the series' highest movement skill ceiling.
Gears 5 leans Execution (Practiced Mastery); Halo 5: Guardians leans Execution (Pure Mechanics).
Halo 5: Guardians leans a little harder on execution, though the gap is small.
Gears 5
Movement tech and shotgun duels remain the core test
Halo 5: Guardians
Thrusters, clamber, and aim raise the ceiling
Halo 5: Guardians leans a little harder on reads, though the gap is small.
Gears 5
Ranked versus rewards out-thinking human opponents
Halo 5: Guardians
Arena ranked play is human-versus-human
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 5: Guardians
Spawns and REQ Warzone randomness
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 5: Guardians
Movement routes and weapon timers
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.