head to head
Gears of War 2 asks more than Grand Theft Auto: San Andreas, and the gap is reads.

Execution · Practiced Mastery
The sequel adding Horde mode to the campaign-plus-versus cover shooter formula.

Optimization · Practiced Mastery
A sprawling open-world crime epic with RPG-style character stats across a whole state.
Gears of War 2 leans Execution (Practiced Mastery); Grand Theft Auto: San Andreas leans Optimization (Practiced Mastery).
Gears of War 2 leans clearly harder on execution.
Gears of War 2
Gnasher battles and movement tech set the ceiling
Grand Theft Auto: San Andreas
Flying school and drive-bys need decent hands
Gears of War 2 leans clearly harder on reads.
Gears of War 2
Human opponents in versus drive prediction play
Grand Theft Auto: San Andreas
Gang AI is scripted, never adaptive
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
Grand Theft Auto: San Andreas
Territory wars and traffic add light randomness
Grand Theft Auto: San Andreas leans a little harder on optimization, though the gap is small.
Gears of War 2
Power-weapon timing and positioning are solvable
Grand Theft Auto: San Andreas
Stats, gym, weapon skill, and routes reward systematic play
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.