head to head
The Jackbox Party Pack 9 asks more than Garry's Mod, and the gap is reads.

Reads · Light / Casual
Source-engine sandbox whose community modes range from building to social deduction.

Reads · Bluff & Deduction
A party-game collection built on reading friends and comedic bluffing.
Garry's Mod leans Reads (Light / Casual); The Jackbox Party Pack 9 leans Reads (Bluff & Deduction).
Garry's Mod leans a little harder on execution, though the gap is small.
Garry's Mod
Varies by mode; base sandbox is undemanding
The Jackbox Party Pack 9
minor typing/drawing speed in some games
The Jackbox Party Pack 9 leans clearly harder on reads.
Garry's Mod
Social deduction modes like TTT involve humans
The Jackbox Party Pack 9
reading fellow players' humor and bluffs
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Garry's Mod
Hidden roles and player chaos in popular modes
The Jackbox Party Pack 9
prompt randomness affects rounds
Garry's Mod leans a little harder on optimization, though the gap is small.
Garry's Mod
Contraption building rewards systems knowledge
The Jackbox Party Pack 9
minor strategic answer choices
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.