My gerbil is huge, pear-shaped, and nesting. Could she be pregnant?

It definitely sounds like Red may be a pregnant female, but he should be seen by a vet if he is not eating/drinking or urinating/defecating normally. Though it is unlikely, this drastic increase in size could be due to a growth within the abdomen or gastrointestinal disease. You can tell if a gerbil is a male or female by looking at the distance between the anus, located just under the tail and the genitals. In males, this distance is longer than in females. If the distance between Red's anus and genitals is shorter than his cagemates, he is most likely a female. The length of a gerbil's pregnancy is about 24 days, and it sounds like Red is nearing the time to give birth. As long as Red is eating, urinating, and defecating normally, you should not need to intervene with the process. Try to interfere with the new family as little as possible, and only spot-clean the cage, as gerbils are easily stressed and may eat their young. The mother and pups should also have free access to food and water. Keep in mind that gerbils can mate immediately after giving birth and become pregnant again, so consider separating Red from the cagemate as soon as possible if (s)he is pregnant.