Translation:Các con gà
The classifier "con" is used mostly for animals. e.g. CON gà (chicken), CON mèo (cat), CON chó (dog), CON ong (bee), etc.
However, "con" can also be used for other objects. e.g. CON dao (knife), CON trăng (moon), CON sông (river), CON tàu/thuyền (boat/ship), etc.
"Các" and "con" are not the same part of speech so you can't really compare them.
"Các" is a determiner used to turn a singular noun into a plural one. e.g: con gà (chicken) -> CÁC con gà (chickenS).
"Con" is a classifier used before a noun so that you can "classify" it based on its physical/non-physical appearance or quantity. e.g: CON gà to (big chicken); một CON ong nhỏ (a tiny bee), ba CON mèo (three cats).
You need to add either "các" or "những" right before an uncountable noun to turn it into a plural one. And for most cases, "các" and "những" are used interchangeably.
However, there are other cases that you have to choose between the two words. Those ones are so complicated that even native linguists still argue. :(
Anyway, feel free to ask a specific case when you can't decide which word to use. We'll try to explain it to you the best we can :)