Literally, it means there is a cat in the bag.
The real meaning is that something is not right.
This comes from another saying:"Cuidado que no te den gato por liebre!" Or, 'be careful they don't give you a cat instead of a hare' Which means, of course, to trick a person.
So, if someone says that there is a 'gato entre el macuto" be cautious, because something is not right.
HB