Gamasjene dekker fjellstøvlene.

Because -ene is the usual definite plural ending for many en-words in Bokmål.

So here you have:

• en gamasje → gamasjer → gamasjene
• en fjellstøvel → fjellstøvler → fjellstøvlene

In English, you normally add a separate word, the.
In Norwegian, definiteness is usually built into the noun ending instead.

Because Norwegian usually puts the definite article at the end of the noun:

• støvel = boot
• støvelen = the boot
• støvler = boots
• støvlene = the boots

So gamasjene already means the gaiters, and fjellstøvlene already means the mountain boots / hiking boots.

A separate determiner can appear when there is an adjective:

• de høye gamasjene = the tall gaiters
• de tunge fjellstøvlene = the heavy hiking boots

That is often called double definiteness.

Dekker is the present tense of å dekke, which means to cover.

Examples:

• jeg dekker = I cover
• du dekker = you cover
• han dekker = he covers
• de dekker = they cover

Unlike English, Norwegian does not change the present-tense verb depending on the subject. The form dekker stays the same.

Because Norwegian normally writes compound nouns as one word.

So:

• fjell = mountain
• støvel = boot
• fjellstøvel = mountain boot / hiking boot

Then the definite plural ending is added to the whole compound:

• fjellstøvler = mountain boots / hiking boots
• fjellstøvlene = the mountain boots / the hiking boots

This is very common in Norwegian. English often uses two words where Norwegian uses one.

The dictionary forms are the indefinite singular forms:

• en gamasje
• en fjellstøvel

Both are en-words in Bokmål. That affects their article and many of their endings.

Typical forms:

• en gamasje / gamasjen / gamasjer / gamasjene
• en fjellstøvel / fjellstøvelen / fjellstøvler / fjellstøvlene

Because gaiters and hiking boots are usually thought of as things that come in pairs.

So it is very natural to say:

• gamasjene = the gaiters
• fjellstøvlene = the hiking boots

If you were talking about just one of each, you could use the singular:

• Gamasjen dekker fjellstøvelen.

That is grammatically fine, but less likely in an ordinary real-world situation.

Yes. This is normal subject–verb–object word order:

• Gamasjene = subject
• dekker = verb
• fjellstøvlene = object

So the sentence structure is straightforward.

It also fits the general Norwegian V2 pattern, where the finite verb is in the second position in a main clause. Since the subject comes first here, the verb naturally comes right after it.

For example:

• I dag dekker gamasjene fjellstøvlene.

Here I dag is first, so the verb dekker still stays in second position.

No. Å dekke can take a direct object.

So:

• dekke noe = cover something

That means fjellstøvlene can come directly after the verb, with no extra preposition.

Compare:

• Gamasjene dekker fjellstøvlene. = The gaiters cover the hiking boots.

This works much like English cover in cover the boots.

Yes. Literally, fjellstøvel is built from:

• fjell = mountain
• støvel = boot

So a very literal translation is mountain boot.
But in natural English, hiking boot is usually the better equivalent.

That kind of small difference is common with compound nouns: the parts are clear, but the most natural translation may use a different everyday expression in English.