Η πύλη για αυτή την πτήση είναι η δώδεκα.

A useful breakdown is:

• Η πύλη = the gate

  • η is the feminine singular definite article
  • πύλη is a feminine noun
  • this is the subject of the sentence

• για αυτή την πτήση = for this flight

  • για = for
  • αυτή = this
  • την πτήση = the flight
  • this whole phrase modifies πύλη

• είναι = is

  • 3rd person singular of είμαι = to be

• η δώδεκα = twelve / number twelve

  • this identifies which gate it is

So the structure is basically:

The gate + for this flight + is + gate twelve.

After για in Modern Greek, you normally use the accusative case.

So:

• η πτήση = nominative, the flight
• την πτήση = accusative, the flight

That is why the sentence has:

• για αυτή την πτήση
• not για αυτή η πτήση

A native English speaker often expects prepositions to work differently, but in Modern Greek many common prepositions, including για, take the accusative.

In Greek, demonstratives like this and that usually go together with the definite article.

So Greek commonly says:

• αυτό το βιβλίο = this book
• αυτή η πύλη = this gate
• αυτή την πτήση = this flight in the accusative

This is completely normal Greek.
So even though English just says this flight, Greek typically uses the pattern:

demonstrative + article + noun

In Modern Greek, the feminine singular accusative form of αυτή is very often written and said as αυτή.

So both of these may be encountered:

• αυτή την πτήση
• αυτήν την πτήση

In everyday standard Greek, αυτή την πτήση is very common and natural.

So if you were wondering why the demonstrative does not seem to show the accusative clearly, that is normal: the article την already makes the case very clear.

In Modern Greek, the final -ν in words like την is kept before certain sounds, including π.

Since πτήση starts with π, we write:

• την πτήση

This helps pronunciation and follows the standard spelling rule.

You will notice the same thing in phrases like:

• την πόρτα
• την πόλη
• την πρώτη φορά

The article η is there because the number refers back to η πύλη.

In other words, η δώδεκα means something like:

• gate twelve
• literally, the twelve one, with gate understood

Because πύλη is feminine, the article is feminine too:

• η πύλη
• η δώδεκα

This is a common Greek way of identifying numbered things, especially when the noun is understood from the context.

No, δώδεκα itself does not change here.

Greek numbers like δώδεκα are generally indeclinable in everyday use, so the number stays the same. What shows gender or case is often the article or the noun around it.

That is why you get:

• η δώδεκα for gate twelve
• not a special feminine form of δώδεκα

So the important agreement is really carried by η, not by δώδεκα.

Yes. Greek word order is more flexible than English word order.

This sentence is perfectly natural, but Greek could also express the same idea in other ways depending on emphasis. For example:

• Η πύλη είναι η δώδεκα για αυτή την πτήση.
• Για αυτή την πτήση, η πύλη είναι η δώδεκα.

In an airport, you might also hear a more direct version such as:

• Η πτήση αναχωρεί από την πύλη δώδεκα.
  = The flight departs from gate twelve.

So the exact order can shift, but the original sentence is a clear and correct full sentence.

A few pronunciation notes:

• πύλη = roughly PEE-lee
• πτήση = roughly PTEE-see
• αυτή = roughly af-TEE
• δώδεκα = roughly THO-the-ka in standard Modern Greek pronunciation

A couple of specific tips:

• In πτήση, the cluster πτ is pronounced clearly, though in fast speech the π may sound light.
• The accent mark shows where the stress goes:
  • πύλη → stress on the first syllable
  • πτήση → stress on the first syllable
  • αυτή → stress on the second syllable
  • δώδεκα → stress on the first syllable

Yes, it is natural and correct, especially as a full textbook-style sentence.

But in real airport language, Greek often becomes shorter or more formulaic. You might also hear:

• Η πτήση σας αναχωρεί από την πύλη δώδεκα.
  = Your flight departs from gate twelve.

• Πύλη δώδεκα.
  = Gate twelve.

• Η πύλη για την πτήση σας είναι η δώδεκα.
  = The gate for your flight is twelve.

So your sentence is good Greek, but real announcements may use slightly different phrasing depending on style and context.