Sorry, but I disagree with the suggestion.
When used correctly about intelligence, "gifted" refers to how a person's brain works. It is something that is permanent and will affect the person throughout their life just as having a learning disability would affect a person throughout their lives.
It's not bragging when a parent with an underachieving gifted student says their kid is "gifted." That info is important because it indicates that the student doesn't have low grades because of an inability to learn the work, but because of something else.
For instance, a parent may realistically be concerned about a gifted student who is holding a 3.0 unweighted gpa while carrying 7 AP courses that s/he never studies for. A parent of a smart, but not intellectually gifted student who is concerned about their kid who is studying hard to get a 3.0 would have completely different issues going on. The gifted student may be lazy or overly perfectionistic and/or may be blowing off school because even an AP-intensive courseload is too easy for them. The smart student may be carrying too difficult a course load.
Here are some of the ways that giftedness may affect people even in adulthood:
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CHARACTERISTICS OF THE GIFTED
Although of course all gifted individuals are unique, they do share certain characteristics. Some of these are present naturally, others have come into existence gradually through interaction with the environment. Cause and effect can therefore not always be distinguished from each other.
Speed of thinking. Gifted individuals think more quickly than others. They make many mental switches, associate rapidly and give the impression that they jump from one subject to the next.
High sensitivity. A higher development potential often is accompanied by high sensitivity7. This high sensitivity manifests itself in different areas: psychomotoric, sensorial, intellectual, imaginative, emotional,7 and can resemble ADHD.
Over-stimulation of the senses manifests itself auditively (machines, radios, smacking lips), visually (light sources) or sense of touch (certain fabrics, labels in clothing, or touching). Currently, there is a high level of interest being shown in high sensitivity in general8.
Introversion. The internal world of the gifted is particularly well-developed. They are quickly and easily hurt, which is why they tend to keep people at a distance. Some avoid parties and suchlike since the topics of conversation do not interest them. This can resemble autism9. Introversion can also arise through having the feeling of being rejected. People with high IQs would seem to have difficulty meeting like-minded people, which can quickly lead them to become isolated10.
Emotional development. Many gifted individuals feel emotions strongly. But because cognitive thinking dominates and provides safety, emotional development remains relatively underdeveloped. They have difficulty in linking feelings and reason. This can be reinforced when an individual has felt lonely from a young age. For example, when the environment does not acknowledge or recognize the child as being gifted. Fortunately, the emotional development of many gifted individuals has progressed well.
Creativity. The thought processes of the gifted differ from those of average intelligence: they are more global in nature and with a strong capacity for imagination. Averagely intelligent people can often not follow their train of thought. They can identify patterns quickly, so that they can, for example, predict trends successfully. They can often draw conclusions intuitively11. This creativity is often frustrated by the regular education system.
Independence. The forming of judgments and opinions often takes place autonomously. They are non-conformist and therefore display what teachers easily label as ‘inappropriate behavior’11. This independence accompanies the creativity mentioned above. They often have an aversion to non-democratic authority12.
Perfectionism. Perfectionism is often accompanied by having too high expectations of others, but also with shame, guilt feelings and feelings of inferiority through not being able to meet their own high expectations 9,13. This leads to tension and occasionally ‘paralysis’.
Learning style. The learning style of the gifted is often exploratory. They have an extreme dislike of learning lists, they find it uninteresting and become bored12. Often, they do not understand the teacher’s questions or the questions in the text books, because they are looking for things that aren’t there. This leads to frustration. Some gifted adults lack basic knowledge but have a lot of knowledge in areas that they are interested in. They often fail to develop learning strategies because they never learn from their failures9.
Fear of failure and under-performing. If their intelligence is not stimulated, children often develop bad working habits14. They sometimes think that they are stupid, become afraid of failure and start under-performing9. Their motivation to learn decreases. This can result later in frustrations and disappointments in their career...."
SENG: Articles & Resources - Gifted Adults in Work
As for "brilliant," in some cases, the term is justified and provides an accurate picture of a person's abilities. When used correctly, it is no more bragging or inappropriate than would be calling, for instance, Tiger Woods a brilliant athlete.
And, of course, just as is the case with giftedness, brilliance doesn't guarantee success or happiness nor does lack of brilliance or giftedness guarantee failure. Hard work, motivation, and other factors are very important.