head to head
Neither is simply harder - they are hard in different places. Gabriel Knight: Sins of the Fathers - 20th Anniversary Edition demands more variance; Portal demands more execution.

Optimization · Pure Optimization
Methodical clue-gathering through a fixed voodoo mystery with single-solution puzzles.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Gabriel Knight: Sins of the Fathers - 20th Anniversary Edition leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Gabriel Knight: Sins of the Fathers - 20th Anniversary Edition
Pointing and clicking - no physical demand
Portal
Portal placement timing modest.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Gabriel Knight: Sins of the Fathers - 20th Anniversary Edition
A scripted mystery with no adaptive adversary
Portal
No adversary modeling player.
Gabriel Knight: Sins of the Fathers - 20th Anniversary Edition leans clearly harder on variance.
Gabriel Knight: Sins of the Fathers - 20th Anniversary Edition
Clues are hidden information until investigated
Portal
Deterministic chambers; no randomness.
Portal leans clearly harder on optimization.
Gabriel Knight: Sins of the Fathers - 20th Anniversary Edition
Day-structured puzzle chains have one intended path
Portal
Pure puzzle-solving optimization.
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.
full breakdown
Gabriel Knight: Sins of the Fathers - 20th Anniversary Edition →
full breakdown
Portal →