The answer is: unity.
The correct word that fills in the blank, in order for it to be grammatically correct, needs to be a noun.
Mahatma Gandhi was an Indian activist who, during the 20th century, led the movement of Indian Independence from British rule. He was characterized by advocating nonviolent civil disobedience, which eventually proved a very successful and exemplary decision. He strongly advocated, also, for unity amongst the people of India.