The Gloss field does not allow embedded writing systems (or embedded styles), since it is normally just one word, or a few words, that would serve as a “translation equivalent” of the vernacular word. Whatever is in the Gloss field needs to be suitable to use in glossing an Interlinear text, since that is really the primary purpose of that field, although it can also serve as a fall-back for the meaning of a word in the dictionary.
Something that is more of a definition rather than a translation equivalent would normally be in the Definition field. Thus, the Definition field allows for embedding other writing systems and styles.
Since there is a “fallback” in the dictionary configuration, you can use the Gloss field for translation equivalents, and the Definition field for more explanatory definitions. When you configure the dictionary, if you have the box for “Definition (or Gloss)” ticked (it is by default), then FLEx will attempt to show the Definition if that is filled in. If it is not, then it will show what is in the Gloss field. For the Dictionary, it is not crucial that you use only one of those fields.