Gibberlings aren't sapient.
Demons etc. are supernatural entities designed solely for the purpose of carrying out evil, not just... people.
I get that, to you, orcs aren't people, but unfortunately since at least the early 1980s, D&D's official writers have frequently treated them as if they were. So they are and have been for a long time.
It's been answered a lot of times man.
Granted for gibberlings (I haven't seen them since Baldur's Gate video game, are they still a thing?) but I disagree that demons/devils are "designed solely for the purpose of carrying out evil". That would be "always evil". And yet, they are not, canonically not since celestials can fall and be demons be redeemed. Such stories predates D&D and are "canonical" parts of D&D. I'd say the exact same problem exist with demons and orcs: either you see them as non-people, and you can ascribe any behaviour to them, or you consider them to be free willed and they are people and shouldn't be treated differently than other people. I think many people are OK with kiling demons is because they are being culturally indoctrinated that "demons = bad guys".