Step 1: Chronological approach: Trace Gandhi's contribution movement by movement, in order: Champaran/Kheda (1917–18) → Non-Cooperation (1920) → Civil Disobedience/Dandi March (1930) → Quit India (1942).
Step 2: What each phase added: Champaran/Kheda proved Satyagraha could work in India; Non-Cooperation first made the freedom struggle a true mass movement; Civil Disobedience (Salt March) internationalized the movement's visibility; Quit India forced the final, decisive confrontation with British rule.
Step 3: The non-political dimension: Alongside agitation, his constructive programme (khadi, anti-untouchability work, Hindu-Muslim unity) built social and economic self-reliance as a parallel track to political demands.
Final Answer: Gandhi's contribution spans early local Satyagrahas, three escalating mass movements, and a constructive social programme, together transforming the freedom struggle into a broad-based non-violent mass movement.