How can you tell if a Gerbil is overweight?
They look fat. If they appear to be pregnant and you know they aren't (especially if it's a guy), then they are too fat. If they look like they have eaten a pear whole that is a sign, too.
I can't say a normal weight is ____grams because you might have a large gerbil or a small one. Generally it is an appearance if your gerbil has a pot belly he's probably a bit overweight but being underweight can also be a serious problem with rodents in general. If the worst happens and your gerbil gets a cold or just isn't quite feeling well they can drop weight at an alarming rate.
Most gerbils will LOVE seeds as they contain a lot of fat, which is why it is best to offer them as a treat, as putting them in their food can cause them to selective feed and become overweight. Your gerbil probably sees the seeds you give them as some kind of "treasure" and so will hide them so no other gerbil can get to them.
You can only tell if a gerbil is pregnant a few days before birth. At that time, she will start to show by a swelling in the tummy region. She will also change behaviors, such as starting to build nests a lot more or hoarding food. If your female gerbil has been in contact with a male gerbil and is over 10 weeks old, then chances are she is probably pregnant.