Long-term complications of diabetes develop gradually.
The longer you have diabetes — and the less controlled your blood sugar — the higher the risk of complications. Eventually, diabetes complications may be disabling or even life-threatening. In fact, prediabetes can lead to type 2 diabetes. Possible complications include:

Heart and blood vessel (cardiovascular) disease. 

Diabetes majorly increases the risk of many heart problems. These can include coronary artery disease with chest pain (angina), heart attack, stroke and narrowing of arteries (atherosclerosis). If you have diabetes, you're more likely to have heart disease or stroke.

Nerve damage (neuropathy). Too much sugar can injure the walls of the tiny blood vessels (capillaries) that nourish the nerves, especially in the legs. This can cause tingling, numbness, burning or pain that usually begins at the tips of the toes or fingers and gradually spreads upward.

Damage to the nerves related to digestion can cause problems with nausea, vomiting, diarrhea or constipation. For men, it may lead to erectile dysfunction.

Kidney damage (nephropathy). The kidneys hold millions of tiny blood vessel clusters (glomeruli) that filter waste from the blood. Diabetes can damage this delicate filtering system.

Eye damage (retinopathy). Diabetes can damage the blood vessels of the eye (diabetic retinopathy). This could lead to blindness.
Foot damage. Nerve damage in the feet or poor blood flow to the feet increases the risk of many foot complications.

Skin and mouth conditions.  Diabetes may leave you more prone to skin problems, including bacterial and fungal infections.

Hearing impairment. Hearing problems are more common in people with diabetes.<

Alzheimer's disease.  Type 2 diabetes may increase the risk of dementia, such as Alzheimer's disease.

Depression. Depression symptoms are common in people with type 1 and type 2 diabetes.

Complications of gestational diabetes

Most women who have gestational diabetes deliver healthy babies. However, untreated or uncontrolled blood sugar levels can cause problems for you and your baby.

Complications in your baby can be caused by gestational diabetes, including:
Excess growth. Extra glucose can cross the placenta. Extra glucose triggers the baby's pancreas to make extra insulin. This can cause your baby to grow too large. It can lead to a difficult birth and sometimes the need for a C-section.

Low blood sugar. Sometimes babies of mothers with gestational diabetes develop low blood sugar (hypoglycemia) shortly after birth. This is because their own insulin production is high.

Type 2 diabetes later in life. Babies of mothers who have gestational diabetes have a higher risk of developing obesity and type 2 diabetes later in life.

Death. Untreated gestational diabetes can lead to a baby's death either before or shortly after birth.

Complications in the mother also can be caused by gestational diabetes, including:
Preeclampsia. Symptoms of this condition include high blood pressure, too much protein in the urine, and swelling in the legs and feet.

Gestational diabetes. If you had gestational diabetes in one pregnancy, you're more likely to have it again with the next pregnancy.

sources from mayoclinic.org & elixir-finland.org