Option 1: The statement is correct. The Gandhara School of Art did owe its origin to the Indo-Greek rulers, but the main patrons of the school were the Kushans, with Kanishka being a particularly important patron.

Option 2: The statement is incorrect. Rich carving, elaborate ornamentations, and complex symbolism were indeed the main features of the Gandhara sculpture. The artists of the Gandhara School were known for their intricate and detailed sculptures, often depicting scenes from the life of the Buddha and other religious figures.

Option 3: The statement is correct. The Graeco-Roman architectural influence did have an impact on the structure of Buddhist Stupas. This can be seen in the incorporation of architectural elements such as columns and arches into the design of some Buddhist Stupas.

Option 4: The statement is incorrect. The artists of the Amravati School of Arts did not mainly use white marble. Instead, they primarily worked with limestone, creating sculptures often adorned with intricate carvings and elaborate detailing.

In conclusion, option 2 is the incorrect statement as rich carving, elaborate ornamentations, and complex symbolism were indeed the main features of the Gandhara sculpture.