Learn Portuguese (Portugal): Dado que gostas de nozes, comprei as que tu adoras. - Given that you like walnuts, I bought the ones that you love.

Here, as acts as a demonstrative pronoun meaning "the ones" (referring to the feminine plural nozes). While you could theoretically say aquelas que, using the definite article o/a/os/as right before que (or de) is the standard, native-sounding way to say "the one(s) that" in Portuguese.

Why is gostas in the indicative rather than the subjunctive after dado que?

The phrase dado que means "given that" or "since." Because it is used to state an established fact or reason, it triggers the indicative mood. It functions just like the conjunctions já que and visto que.

Why is there no definite article in gostas de nozes?

Leaving out the article makes it a general statement about a category: "you like walnuts" as a whole. If you used das nozes (de + as), it would mean you like a specific batch of walnuts, such as "the walnuts from that particular store."

Does nozes refer to nuts in general, or specifically to walnuts?

It refers specifically to walnuts. The singular form is noz. If you want to talk about nuts in general, the phrase you need is frutos secos (literally "dry fruits").