Learn Italian: Darai l'eredità all'erede? Sì, gliela darò. - Will you give the inheritance to the heir? Yes, I will give it to him.

It is a double object pronoun. We combine the indirect object gli (to him) and the direct object la (it, referring to the feminine noun l'eredità). When gli combines with lo, la, li, or le, it adds an e to become glie-, merging into one single word: gliela.

If the heir were female, would gliela change?

No, it stays exactly the same. When forming combined pronouns, the feminine indirect object le (to her) changes to glie-, merging with the direct object exactly like the masculine gli does. So gliela translates to both "it to him" and "it to her".

Why doesn't the future tense of dare follow the regular -are verb rules?

Normally, -are verbs change the a to an e in the future tense (for example, parlare becomes parlerò). However, dare (to give), stare (to stay/be), and fare (to do/make) are common exceptions. They keep the a in their stem, which is why we see darai and darò here instead of derai or derò.