When you're pregnant your hormones change drastically, throwing everything out of whack. You also gain weight, causing insulin resistance. If your hormones and weight go back to normal, these triggers are gone and the diabetes may go into remission. This is an oversimplicfication, but in essence, its a return to normal levels of hormones and weight that helps.
Not all women do experience a remission, some stay diabetic after delivery. Statistically, most women that experience diabetes during pregnancy will become a full blown type 2 diabetic within 5 years. (not all, but most will). The genes are there, or you wouldn't get gestational diabetes in the first place.
With every baby, women usually gain some weight that stays put, and their insulin resistence increases. They also don't get enough sleep and their health suffers as they take care of the new baby and any subsequent children. there are all sorts of stresses with pregnancy and raising a family that can contribute. Aging also increase the likelyhood of getting type 2 for anyone who has the genes for it (at this time there are at least 38 known genes that can trigger type 2 when conditions are right). Its hardly surprising that most women with gestational diabetes end up with type 2 eventually. This makes testing very important.
Always get tested for diabetes after pregnancy at 6 and 12 weeks postpartem, and its a very good idea to get tested every year for the rest of your life. Those genes are still there and have already been triggered once. The key to good control is to catch it early and get it under control right away.