Giftedness is more about being developmentally more advanced and having high ability and/or potential in some area. It makes them out of sync with their peers. There are many theories, one of which is Dabrowski’s over excitabilities, but in essence they do function differently from others.
Some gifted students are high performing students but a surprisingly number are not. Many gifted kids struggle with motivation and persistence. True gifted programs deal with the needs of gifted kids development — the so called gifted programs in schools today are more like accelerated classes which have little to do with developmental needs. Giftedness is a special need on the other side of the spectrum.
I think we all know the type: really intelligent kid but amounted to not much in life. (The most well known pop culture portrayal was Good Will Hunting although that’s all fictionalized and not gifted kid is at that level — there are different levels)