Translation:The cat has been adopted by nice people.
Second description for "gentil" on larousse.fr ( http://www.larousse.fr/dictionnaires/francais/gentil_gentille/36613#iYeMXYlmmgs7v4vD.99 ):
"Aimable, complaisant, plein de bons sentiments à l'égard d'autrui ; qui manifeste ce caractère ; délicat : C'est une gentille attention de sa part."
Sounds to me like a perfect description of friendly, so I reported that "friendly" should be accepted.
In fact, words rarely translate directly one to one and even in a single language meanings overlap, or can you mark a clear divide in English were you have to stop using "kind" and have to say "friendly" instead?
The former could be used in cases where the cat is no longer adopted (e.g. it died, they gave it to neighbours when they moved etc.) whereas the latter implies that it is still with the people who adopted it (this is because of the meaning of the English present perfect tense which implies an effect extending to the present. The French passé composé can have either meaning which is why both translations are correct)
"People" is already a plural noun, when referring to individuals: "One person says..." "Lots of people say..." There is an English word, "peoples"; but it refers to multiple groups of people, of, say, different ethnicities: "The middle east is home to many different peoples."
the phrase was adopted is a one off event so the passé compose is appropriate
the use of the imperfect conjugation of être would be appropriate if you were using the pluperfect.
la voiture était lavée sous la pluie - the car was being washed in the rain
La tarte était faite par Anne. - The pie was being made by Anne
Il s'était tu pendant la conférence - He had kept quiet during the lecture.
"A été adopté" can be translated "was adopted" or "has been adopted." It is the simple past tense. "Était adopté" is using "adopté as an adjective (not as a verb strictly speaking) with the verb "être" in the imperfect tense, and is translated "was adopted." As in English, these mean about the same thing but have a slightly different flavor.
Bad news - the "BAGS" rule is not a law. It's a "rule of thumb". Not everything that could conceivably fall into those categories is actually placed before the noun.
'the cat was adopted by some nice folks' gave me a no way Jose. scratch his head "des gens gentils" sounds plain ridiculous with the repetition of "gen" [and bear in mind that a "jean-jean" is slang for a simpleton or naive person.] "des gens/personnes sympathiques/sympa" would be correct and sound better. Bear in mind that "gentil" can be use in a pejorative way sometimes.