What does it mean when a gerbil sqeaks?

Its quite hard to answer this without hearing the noise you are referring to as ferrets make several different squeaks, which mean different things.

Dooking is a clucking noise. It is usually accompanied by the ferret running around and jumping about as he / she is very happy or excited

Screeching / screaming is a high pitched noise and is often accompanied by a quick chattering noise. This noise means the ferret is very scared or in pain. Sometimes ferrets will screech during play because they are very excited.

Hissing sounds similar to a cats hiss. It means that the ferret is angry.