Mohandas Karamchand Gandhi (2 October 1869 – 30 January 1948), also known as Mahatma Gandhi was a central figure in the formation of the modern Indian nation. A Hindu of the merchant caste, he studied law in England and first engaged in the struggle against colonialism in South Africa, where he began to formulate his theory of non-violent resistance, combining traditional Hindu religious commitments with more western concepts of tolerance and religious freedom. 

He is best know for his struggle to free India of British rule through nonviolent means, including tactics such as protest marches and hunger strikes, and was crucial to the task of trying to establish independent India as a pluralistic democracy. He was strongly opposed to the caste system and all forms of religious, gender, and ethnic discrimination, partly as a result of having had close Muslim friends as a child. He objected to the oppression of women and opposed the practice of sati, paving the way for many of the rights women enjoy in India today.

Throughout his life, he remained a strict vegetarian and advocated a simple, non-materialistic lifestyle, with a strongly spiritual focus. He is still revered as a role model by many people in India.

Another effect Gandhi had is to bring awareness to India and its struggle for statehood.  Even today people are so impressed by his ideas and actions that we still remember India's quest for independence and actions taken to become independent of Britain.

I would think that the major impact that Gandhi had on India was to help the country get its independence.  He was not the only one who was responsible for that happening, but he clearly played a major role in that process.