I like "expertise" - but other terms like "ability", "proficiency", "mastery" are also ok. We're talking about a capacity you have, an ability.
"Feat" sounds like something you've
done, a deed you've performed. This doesn't sound like a capacity you can re-deploy on different occasions in different circumstances. For similar reasons, I don't like "gain".
"Trait" and "perk" sound too neutral.
Just to throw in my two cents, on a largely inconsequential matter.