Eu gosto de fruta, especialmente manga.

Because gostar normally takes the preposition de in Portuguese.

So:

• Eu gosto de fruta. = I like fruit.
• Ela gosta de café. = She likes coffee.
• Nós gostamos de música. = We like music.

For English speakers, this is one of those patterns you just memorize:

• gostar de + noun / verb

Examples:

• gosto de manga
• gosto de cozinhar
• gosto de você

So Eu gosto fruta sounds incomplete in standard Portuguese.

You can often leave out eu in Portuguese, because the verb form already shows who the subject is.

So both are natural:

• Eu gosto de fruta, especialmente manga.
• Gosto de fruta, especialmente manga.

Including eu can add clarity, emphasis, or contrast:

• Eu gosto de fruta, mas meu irmão não.

In everyday Brazilian Portuguese, dropping the subject pronoun is very common when the meaning is clear.

In Portuguese, just like in English, a singular noun can be used in a general sense.

So:

• Eu gosto de fruta. = I like fruit.
  This treats fruit as a general category or food type.
• Eu gosto de frutas. = I like fruits.
  This sounds a little more like “fruit in general” as countable kinds/items.

Both are possible, but gosto de fruta is very natural when talking about fruit as food in general.

A similar contrast:

• Eu gosto de carne. = I like meat.
• Eu gosto de legumes. = I like vegetables.

Portuguese often omits the article when talking about things in a general way.

So:

• gosto de fruta = I like fruit
• especialmente manga = especially mango

This sounds natural when you mean the category, not one specific item.

Compare:

• Eu gosto de fruta. = I like fruit in general.
• Eu gosto da fruta que você comprou. = I like the fruit that you bought.

And:

• Eu gosto de manga. = I like mangoes / mango as a fruit.
• Eu comi a manga. = I ate the mango.

So in your sentence, no article is needed because both nouns are being used generically.

Yes, but it changes the feel a little.

Here are some natural options:

• Eu gosto de fruta, especialmente manga.
• Eu gosto de frutas, especialmente manga.
• Eu gosto de frutas, especialmente de manga.

The original sentence is smooth and natural. Adding de before manga is also possible because especialmente can connect nicely to a de phrase in some contexts, but many speakers would simply say:

• especialmente manga
• principalmente manga

So the original is excellent and idiomatic.

Yes, especialmente is a very good match for English especially.

In this sentence:

• Eu gosto de fruta, especialmente manga. means:
• I like fruit, especially mango.

It highlights one favorite within a larger group.

Other similar words are:

• principalmente = mainly
• sobretudo = above all / especially
• em especial = in particular

Examples:

• Gosto de música, especialmente jazz.
• Ela gosta de animais, especialmente gatos.

The comma separates the main statement from the extra detail.

Main idea:

• Eu gosto de fruta

Added specification:

• especialmente manga

It works a lot like English:

• I like fruit, especially mango.

The comma makes the sentence easier to read and sounds more natural in writing. In speech, you would usually make a small pause there.

A simple pronunciation guide is:

• Eu ≈ eh-ooh / often more like yo in fast speech
• gosto ≈ GOHS-too
• de ≈ jee or djee in many Brazilian accents when unstressed?
  More carefully, it is dji-like in many regions before some sounds, but here many learners can think of it as a light dee/dji sound depending on accent.
• fruta ≈ FROO-tah
• especialmente ≈ eh-speh-syal-MEN-chee in many Brazilian accents
• manga ≈ MAHN-gah

A few useful pronunciation notes for Brazil:

• Final o in gosto often sounds like u: gostu
• Final te in especialmente often sounds like chee: especialmenchi
• ã in manga is nasal: not a plain ma, but a nasal mãn

A rough natural Brazilian pronunciation could sound like:

• Eu gostu dji fruta, especialmente mãga.

Yes to both.

Manga is a feminine noun:

• a manga = the mango
• a manga da camisa = the shirt sleeve

So manga can mean:

1. mango
2. sleeve

Usually the context makes it obvious:

• Eu comi uma manga. = I ate a mango.
• A manga da camisa está suja. = The shirt sleeve is dirty.

In your sentence, because it appears after fruta, the meaning is clearly mango.

Yes. If you want a stronger feeling, you could say:

• Eu adoro manga. = I love mango.
• Eu amo manga. = I love mango.
  This is possible, though adoro is often more natural for foods.
• Eu gosto muito de manga. = I like mango a lot.

So:

• gosto de = like
• gosto muito de = like a lot
• adoro = love / adore

You can change the word order, but the meaning or emphasis changes.

Original:

• Eu gosto de fruta, especialmente manga. This means: I like fruit in general, and mango is the standout example.

If you say:

• Eu gosto especialmente de manga. that means: I especially like mango.

That version focuses more directly on mango rather than fruit in general.

And:

• Especialmente, eu gosto de manga. is grammatical, but it is less natural in everyday conversation.

So the most natural alternatives are:

• Eu gosto de fruta, especialmente manga.
• Eu gosto especialmente de manga.

Yes, it sounds natural.

That said, in casual speech you might also hear:

• Gosto de fruta, principalmente manga.
• Eu gosto de frutas, especialmente manga.
• Adoro manga.

Your sentence is clear, correct, and natural in Brazilian Portuguese. It has a neutral, standard feel.