"As he is an only child, he does not like losing."
Translation:Essendo figlio unico, non gli piace perdere.
According to Reverso, the phrase "As he is ..." is more commonly translated as "Come lui è..." or "Dato che...". See: https://context.reverso.net/translation/english-italian/As+he+is
What on earth does being an only child have to do with liking or not liking to lose? WHO likes to lose?
"Since" or "as", so the English question could also be expressed as "since he is an only child". However I agree with JBrenner, especially as this question is in the Gerund section; using "as he is an only child" instead of "Being an only child", there does not appear to be any gerund in this.
I think we can sometimes be mistaken when we want precise consistency of grammatical construction between English and Italian sentences. DL does have annoying inconsistencies, but in this case, I think it is trying to teach us that -ing words in English don't always translate into -endo words in Italian and vice versa. So English -ing constructions can translate to Italian as gerunds, as infinitives, as simple present or imperfect tense verbs, or as something completely different. The same is true in reverse for -endo words. We need to recognise that what looks to us like the closest grammatical cognate in the other language is not always what sounds most natural to a native speaker.
Because you are essentially saying "Being an only child, he does not like losing," not "He is being an only child, he does not like losing." In the first, you do not use "sta", where in the second, you would- though it is very awkward to say that! The first sentence contains two clauses that are related to each other, the second contains two totally independent clauses.
"unico filgio" should be accepted: https://dictionary.reverso.net/italian-english/unico%20figlio
In English it would be the difference between "to be an only child" (essere) and "being an only child" (essendo). The first would really only work if you are talking about the state of being an only child in general, while the second is talking about one specific only child. The rest of the sentence makes it clear that we're talking about one person in particular.
I cannot see any difference in the meaning of these two English sentences either. However, this an exercise on gerunds and the gerund in the English sentence occurs in the second phrase ("losing") whereas in the translation it occurs in the first phrase ("Essendo"). Is the translation only to be governed by the meaning of the sentence or is the structure also to be taken into account?.
A good reason for the variety in translations in this section is that the "gerundio" (-ando, -endo) structure in Italian does not correspond perfectly to the "-ing" form in English. Remember that this is to test the Italian gerundio form, not the English. The true English gerund form (i.e. a verb acting as a noun) is actually most commonly translated in Italian as the infinitive, as "losing" is in this sentence.