Gateadressen står før postnummeret og poststedet.

Because Norwegian usually writes compound nouns as a single word. So gateadresse is one compound meaning street address, and its definite singular form is gateadressen.

This is very common in Norwegian:

• postnummer = postal code
• poststed = post town / postal place

English often uses separate words where Norwegian prefers one compound word.

Those endings mark the definite form, like the in English.

So:

• gateadressen = the street address
• postnummeret = the postal code
• poststedet = the post town

Instead of putting a separate word for the in front of the noun, Norwegian usually adds the definite ending to the noun itself.

Because the nouns have different grammatical genders.

• en gateadresse → gateadressen
• et postnummer → postnummeret
• et poststed → poststedet

So -en is the definite ending for many common gender nouns, while -et is the definite ending for many neuter nouns.

Not literally. In Norwegian, stå is often used about text, information, or items in a certain position.

So Gateadressen står før postnummeret og poststedet means something like:

• The street address appears before the postal code and post town
• The street address comes before...

This is a very natural use of stå in Norwegian.

Here før means before in order or position, not before in time.

So it is talking about the order of parts in an address:

• first the street address
• then the postal code and post town

This is normal Norwegian main-clause word order: subject + verb + the rest.

• Gateadressen = subject
• står = verb
• før postnummeret og poststedet = adverbial/prepositional phrase

Norwegian main clauses follow the V2 rule, which means the finite verb normally comes in the second position.

Poststed is the place name used together with a postal code in a Norwegian mailing address. It is often the town or locality connected to that postal code.

So in an address, postnummeret og poststedet naturally belong together.

Because both nouns are specific parts of the address format, and each noun carries its own definite ending.

In English, you can say the postal code and post town, with one the covering both nouns. In Norwegian, there is no separate the here, so each noun is marked as definite itself:

• postnummeret
• poststedet

Yes. Kommer før is also possible and would be easy to understand.

But står før sounds especially natural when talking about written order, layout, or how information is presented on a page or form. So in this sentence, står før is a very good choice.

Yes, but that would sound more general and less like a specific address format.

Compare:

• Gateadressen står før postnummeret og poststedet.
  = talking about the specific elements in an address layout

• Gateadresse står før postnummer og poststed.
  = more general/category-like, and less natural on its own

So the definite forms fit this sentence better.