Gestational diabetes isn't very common, but is well known and well understood - it occurs in about 2 to 5% of pregnancies. Most of these cases can be managed with careful control of your diet and regular exercise, but in some cases medical treatment is needed as well. If the condition is carefully monitored, it should be safe; pregnant women with this condition should work closely with the midwife or doctor to monitor their blood sugar levels and follow a specifici diet and exercise regime. Insulin should further lower the risk.
The main risk is that the unborn baby may be too large for its stage of development. This will also be closely monitored, and if there is any risk of complications for this or other reasons the health professionals should talk to you about other birth options. For example, it may be advisable to induce labour a little early.
You can get a full overview of gestational diabetes and the possible complications here.