Ráðið kom að gagni.

Ráðið is the neuter noun ráð with the definite article attached.

The basic word ráð can mean things like:

• advice
• a solution
• a plan
• a remedy / measure

Which English word fits best depends on context. In this sentence, it means the thing that turned out to be useful.

The ending -ið is the suffixed definite article for a neuter singular noun.

So:

• ráð = advice / solution
• ráðið = the advice / the solution

Here ráðið is in the nominative singular, because it is the subject of the sentence.

Kom is the 3rd-person singular past tense of koma, meaning to come.

So:

• koma = to come
• kemur = comes
• kom = came

Since ráðið is singular, the verb is singular too.

Because koma að gagni is an idiomatic expression.

Literally, it is something like come to use / come to benefit, but that is not how English normally says it. Natural English translations are:

• was useful
• helped
• came in handy
• proved useful

So this is a phrase you should learn as a whole: koma að gagni.

Að gagni is a fixed phrase meaning of use, beneficial, or helpful.

The noun behind gagni is gagn, which means use, benefit, or good.

So:

• gagn = use / benefit
• að gagni = of use / to good effect / beneficial

In this sentence, it combines with kom to make the full expression kom að gagni.

Gagni is dative singular.

The basic noun is gagn, but after the preposition að, Icelandic normally uses the dative. That is why you get:

• gagn → gagni

This is another reason it is helpful to memorize að gagni as a set phrase.

No. Here að is a preposition, not the infinitive marker.

A quick way to see that is this:

• infinitive-marker að is followed by a verb
  
  • að lesa = to read
• prepositional að is followed by a noun
  
  • að gagni

So in kom að gagni, að belongs to the prepositional phrase að gagni.

Yes. Very often you add a person in the dative.

For example:

• Ráðið kom mér að gagni. = The advice was useful to me.
• Ráðið kom honum að gagni. = The advice was useful to him.
• Ráðið kom þeim að gagni. = The advice was useful to them.

This is a very common pattern.

No. It is the normal neutral order for a main clause:

• Ráðið = subject
• kom = verb
• að gagni = prepositional phrase / complement

So the sentence is structurally very straightforward.

Icelandic can change word order for emphasis, but this version is the plain, ordinary one.

It is a normal, standard expression. It is not strange or overly literary.

You can hear it in ordinary speech and also see it in writing. It may sound a bit more idiomatic than a very direct wording like vera nytsamlegt or vera gagnlegt, but it is completely natural Icelandic.