These two words are synonyms, there is not particular case when you must use "gens" instead of "personnes". Just one thing : - "gens" is always a plural, you can't say "(1) un gens", it's always "des gens" - "personne(s)" can be use as a plural and a singular, then you can say "(1) une personne" or "des personnes"

En effet, gens est toujours au pluriel. Il est selon moi bien moins précis que personnes. http://fr.wiktionary.org/wiki/gens#Nom_commun_1

I don't agree with Rose. They are synonyms, but sometimes you just can't replace one by the other. Like Olivier said, "personnes" is more precise. I'm not sure how to explain so I'll give you an example: You could say "Les gens sont fous." (talking about people in general) ... but you can't say "Les personnes sont folles". It doesn't make sense. People would ask you who you're talking about. On the other hand, if what you want to say is "Ces gens sont fous" (talking about some people in particular), you can also say "Ces personnes sont folles" (it sounds a bit weird but it makes perfect sense).