a fruit-flavoured gelatin dessert made up from a powder.

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Jell-O refers to a specific brand of gelatin dessert. You can transcribe the name (as suggested by others) if it is important to retain the name. From the context, however, it doesn't seem like it is important to retain the name. In the United States, we call any gelatin dessert of this type "jello." A doctor or nurse might tell you to eat jello, but he/she doesn't mean the specific brand. In Jordan/Palestine (and probably in other countries), people usually call it جيلي. I also agree with Liliane's answer (هلام).

See the following:

https://en.wikipedia.org/wiki/Jell-O

https://en.wikipedia.org/wiki/Gelatin_dessert

https://ar.wikipedia.org/wiki/هلام_(حلوى)

هذا اسم منتوج وأعتقد أنه من الأفضل المحافظة على الاسم كما هو.
يمكن أن تكتب:
(حلوى الهلام (جيل- أو