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
The sequel adding Horde mode to the campaign-plus-versus cover shooter formula.

Execution · Pure Mechanics
Multiplayer-focused Halo with the series' highest movement skill ceiling.
Gears of War 2 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 of War 2
Gnasher battles and movement tech set the ceiling
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 of War 2
Human opponents in versus drive prediction play
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 of War 2
Horde spawns add mild randomness
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 of War 2
Power-weapon timing and positioning are solvable
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.