Translation:It is raining.
The verb pluvi is non-transitive, so you can't have viroj in the accusative case with the -n ending.
It's correct to use the verb pluvi with a subject (though it's not necessary), so it's correct to say for example eta pluvo pluvas and thus figuratively it's also correct to say pluvas viroj.
In Esperanto you do not have to make difference between "It is raining (now)" and "It rains (every day)". And normally you do not. You say "Pluvas." for both cases.
In the rare case that it is important to make the difference you can either give the time: Pluvas nun, pluvas daŭre, pluvas ĉiutage.
Or you have the two verb forms. "Estas pluvanta (nun)" and "pluvadas (daŭre, ripete)". But it is certainly not correct to use these forms everytime. They are only for the rare cases where you want to make a special point.
That’s not a gerund, but the adverbial form of the participle to modify the verb. Because the subject of these weather phrases is a not mentioned weather god :-) it is difficult to form a sentence with the same subject. Perhaps like this: Malheliĝas pluvante. – It gets dark while raining.
It is better with a real subject: Mi lernas Esperanton aŭskultante. – I learn Esperanto while listening.
Let’s extract subject, verb and object: The peoples form a family. And this is the answer to how they do it: by understanding each other.
These constructions with adverbial participles replace whole sub-clauses which would have the same subject as the main clause.
Participles can have three functions:
Adverbial: Komprenante unu la alian la popoloj faras ion …
As adjectives: la interkomprenantaj popoloj faras ion …
To form additional verbforms: Se mi estus sciinta – if I had known …/La popoloj estos komprenintaj unu la alian. – The people will have understood each other.
There's also the adjective “pluvial” used in geology and climatology to describe something related to rain or having high precipitation or humidity. So there are pluvial lakes and pluvial floods, oh, and that one time in Earth's history when it rained for two million years, called the Carnian Pluvial Event.