Sentence #1
I remember being taken to the zoo.
This is correct and sounds very normal to native ears. Being taken to the zoo functions as a noun phrase denoting the act in which you were taken to the zoo. Being is a gerund here. The passive construction here omits the subject; the implied subject is me (which a person would only say in order to create unusually strong emphasis).
It might help to look at some comparable sentences where the subject of being taken is explicit:
I remember John being taken to the zoo.
I remember him being taken to the zoo.
Notice that the subject of being taken is in the objective case. That's because it's also the object of remember, the main verb of the sentence.
Sorry, I have to tell you this
There's also an older school of thought that says the previous two sentences are incorrect and should instead be:
I remember John's being taken to the zoo.
I remember his being taken to the zoo.
In this parsing, the object of remember is being taken to the zoo. The subject of the gerund in this construction takes the possessive case (strangely enough).
Most fluent speakers today hear both him being taken and his being taken as correct. That is, people can parse both forms. The first form works by analogy with I helped him learn and the second form works by analogy with I helped his education.
Sentence #2
They are remembered taking me to the zoo.
This is actually correct, just a little unusual. Here, taking is a present participle, not a gerund. Taking me to the zoo modifies they, in the manner of a subject-complement. A comparable sentence might make this clearer:
Football players are usually drawn running with the ball.
In other words, in most pictures of football players, the football player is running with the ball.
The reason your example sentence is unusual isn't because of the grammar, it's because it's a little hard to imagine a situation where people would be remembered that way. But it's certainly possible and the sentence can be understood. Perhaps someone took a photograph of "them" while they were taking you to the zoo, they died long ago, and since then, many people have seen this photograph.
"Gathered" is synonymous with "came together" or "met" or "assembled" or "congregated".
Had gathered would be appropriate if you wanted to stress the sequence of events:
After they had gathered for a meeting, they were killed by militants.
If you wanted to say that they were in a group when they were killed:
They were gathered for a meeting when they were killed by militants.
"they were gathered for a meeting... when ... " would not be interpreted as a passive construction—from context it would not be understood to mean "they were rounded up".
But if you wrote:
They were gathered together and then killed by militants.
then it would be understood to mean that they had been rounded up.
The clearly passive construction is "people were killed by militants".
Best Answer
You are overthinking the passive voice and objects of a transitive verb. To work and to travel could be used both as a transitive and intransitive verb. The characteristics of this kind of verb is it is very flexible in terms of having a direct object or prepositional phrase as its complement. For example,
For nine hours is a prepositional phrase and nine hours could be an adverbial noun. But the question is "why can't nine hours be an object of the verb to work?" They obviously can. Otherwise, you can't passivize the sentence.
All the above examples have so called "whiz deletion (omission of a relative pronoun + the verb to be)" and they are idiomatic expressions. If you contrast,
You could notice that the former phrases are more concise.
Nobody writes "working hours" as "hours that workers spend for working". Why? Because the two words working hours are enough to express the longer phrase.
If you replace to work with to spend, it would be easier to understand.
"Hours spent by employees" would not be concrete enough compared with "Hours worked by employees" because to spend has a broader meaning than to work.
English is a flexible language. You should not care so much about this kind of structure as there is no hard-and-fast rule in terms of passivizing a sentence. There is just a general rule.
This means six guests can sleep in the room. Can you passivize this sentence to:
A native speaker would say, "what?" Not all the transitive verbs could be passivized. In addition, an object that looks like an adverbial noun could be passivized. Why? It could be considered as an object of a transitive verb.