A true sign of power is the ability to influence and coerce without the need for violence or military force. Anybody can amass an army and force people to do their bidding under the threat of death, but a truly powerful being can influence minds without picking up a weapon.
Gandhi was the undisputed leader of 390 million Indians and he never picked up a sword or a gun. He changed the way an entire generation thought and perceived the world around them. He was responsible for the birth of a nation and his teaching influenced leaders like Martin Luther King Jr. and Nelson Mandela. The source of Gandhi's strength was his influence. He held no offices or titles, but simply the will of the people. His intellectual leadership did not only influence his contemporaries but also generations that followed. Intellectual and ideological contributions often last much longer than arbitrary wealth or power over a kingdom.
Even if Gandhi had a strong influence over the entire population of India (which is highly debatable), others on this list controlled populations far larger. Additionally, Gandhi was merely a vessel for a sentiment felt by many people. He was the voice and the face of a movement that required the peoples' will to mobilize. Gandhi would have been unable to exercise any power outside of what the people wanted, so his power was reliant on their ideological alignment. Most importantly, his "power" was too abstract to be compared to other historical figures. He controlled no land, wealth, and military. He would have been unable to take control over any place or people by force, and his movement was defenseless to external physical acts of power.
The most powerful force on earth is the love of the public. Gandhi secured the support of the entire nation of India and could, therefore, shape ideology.
[P1] The most powerful political force is grassroots support. [P2] Gandhi could mobilize grassroots support better than anyone on earth. [P3] Gandhi's ideological contributions will contribute to human society long after his death.
Rejecting the premises
[Rejecting P1] Gandhi's support was predicated on the will of the people, not his individual power. [Rejecting P2] Other historical figures could mobilize support across a larger population than Gandhi. [Rejecting P3] Ideological contributions do not necessarily reflect on the "power" of the individual in their lifetime.