Weight is actually a force, in case you're a non-physisist who didn't know that. it's the force which gravity exerts upon one's mass, meaning it is measured on a spectrum. if, as emerald's pokedex entry states, it doesn't feel the pull of gravity, why does it weigh 48 KG?
It's probably best if you read the whole sentence:
It apparently does not feel the pull of gravity because it supports itself with psychic power. Emerald
It's being pulled by gravity, but it's psychic abilities allow it to levitate and is strong enough so it doesn't even feel it.
I know Physics. Weight and mass is different. But I'm 'no physicist" tho. The fact that it says kilogram in the pokedex means that it's in mass. So it does not tell you if it's either affected by gravity or not. If you're a physicist then, you should also know that the unit, if gardevoir is affected by gravity, is in Newton.