Autoimmune Diseases:
Autoimmune diseases are a group of disorders in which the immune system mistakenly attacks the body's own healthy cells and tissues. There are several autoimmune diseases, and among the given options, Type I Diabetes Mellitus and Pernicious Anemia are autoimmune diseases.

Explanation:

Type I Diabetes Mellitus:
- Type I diabetes mellitus, also known as insulin-dependent diabetes or juvenile diabetes, is an autoimmune disease.
- In this condition, the immune system mistakenly attacks and destroys the insulin-producing cells (beta cells) in the pancreas.
- As a result, the body is unable to produce enough insulin, leading to high blood sugar levels.
- This condition usually occurs in childhood or adolescence.

Pernicious Anemia:
- Pernicious anemia is another autoimmune disease.
- It is characterized by the body's inability to absorb vitamin B12 from the diet, leading to a deficiency of this essential nutrient.
- The immune system produces antibodies that attack the cells in the stomach lining that produce a protein called intrinsic factor.
- Intrinsic factor is necessary for the absorption of vitamin B12.
- Without sufficient vitamin B12, the body is unable to produce enough healthy red blood cells, leading to anemia.

Other Options:

Type II Diabetes Mellitus:
- Type II diabetes mellitus is not an autoimmune disease.
- It is a metabolic disorder characterized by insulin resistance, where the body's cells become less responsive to the effects of insulin.
- While there is a genetic component to Type II diabetes, it is not primarily caused by autoimmune mechanisms.
- Lifestyle factors, such as obesity and physical inactivity, play a significant role in the development of Type II diabetes.

Gestational Diabetes:
- Gestational diabetes is a form of diabetes that occurs during pregnancy.
- It is characterized by high blood sugar levels in pregnant women who did not previously have diabetes.
- Gestational diabetes is not considered an autoimmune disease.
- It is believed to result from hormonal changes and increased insulin resistance during pregnancy.

Conclusion:
In conclusion, among the given options, Type I Diabetes Mellitus and Pernicious Anemia are autoimmune diseases. Type II Diabetes Mellitus and Gestational Diabetes are not autoimmune diseases but have different underlying causes.