"They are good people."
Translation:Ce sont de bonnes personnes.
It's a grammar rule for "modified nouns" - see http://french.about.com/library/weekly/aa032500c.htm
Ils sont bons / Elles sont bonnes - adjective on its own
ce sont mes parents - article + noun
ce sont de bonnes personnes - adjective + noun
The position depends on the meaning. Here is a lesson on adjective placement: http://french.about.com/od/grammar/a/adjectives_fickle.htm
That article is really helpful - glad to know what these adjectives depending on their placement. Bon placed after is good in the moral sense, and I see also bien can be used: Ce sont des gens bien - see: http://french.about.com/od/vocabulary/a/bon.htm
I am also wondering what are the nuances of "un homme bien" as it seems to be used in the romantic sense. If I say a man is "un homme bien" will people think I fancy him?
I'm not an expert in these matters, but from what I gather "gens" is used to refer to a generic unspecific group of people and "personnes" is used to refer to a specific group of people. Although we don't have a context for this sentence they are good people does seems to suggest the speaker has specific people in mind, hence "personnes" and not "gens"
EDIT: And I assume you'd still need the article "de" so that might be why your sentence was marked wrong.
google translate says gens in feminine. ignoring the fact that I was cheating (trying to figure out the gender, not the word itself) I'm now wondering whether google translate was wrong or the answer here was wrong. It's a bit of a toss-up but I'm leaning towards google translate being wrong...both considering it so often is and that my first instinct was 'les gens bons'...I miss so many answers by ignoring my first instinct.
The meaning of "gens" doesn't change.
Even stranger it doesn't actually change gender. So even if "gens" is preceeded by an adjective (feminine form) any adjective or participle that follows "gens" in the same sentence will be in the masculine form.
So even within a single sentence "gens" can be treated in one part of the sentence as if feminine but in another part of the sentence be treated as if masculine.
E.g. "Les vielles gens sont attachés à leurs racines" (Old people are close to their roots) - As you will know if "gens" is feminine then we should have "attachées" and yet we have feminine "vielles" but masculine "attachés"
As I say "gens" is an interesting and strange word ;)
I can see a few things that might be the issue. First, if I have my French grammar correct, you don't use the word 'ils' in these sentence structures. The noun 'personne' used to describe the people being talked about is modified by 'des' and 'gentilles' in your example, thus requiring the word 'ce' instead of 'ils'. I don't have a link for more information, but it's worth looking into, and maybe someone else will provide one if they have it.
Second, the word 'gentille' literally means 'nice'. A nice person is not the same thing as a good person. My real estate agent was always really nice to me, but he was dishonest and lazy and just tried to rush me through the process so he could get his commission. (Found out too late that he doesn't have the best reputation) So I would say, that while he was a nice real estate agent, he wasn't a good one.