This principle maintains that when the human mind (perceptual system) forms a percept or "gestalt", the whole has a reality of its own, independent of the parts. The original famous phrase of Gestalt psychologist Kurt Koffka, "The whole is other than the sum of the parts" is often incorrectly translated as "The whole is greater than the sum of its parts", and thus used when explaining gestalt theory, and further incorrectly applied to systems theory. Koffka did not like the translation. He firmly corrected students who replaced "other" with "greater". "This is not a principle of addition" he said. The whole has an independent existence.

Based on this passage, who said the whole is greater than the sum of its parts? Let's give stream of consciousness first:
To answer the question, consider the following: The original famous phrase of Gestalt psychologist Kurt Koffka, "The whole is other than the sum of the parts" is often incorrectly translated as "The whole is greater than the sum of its parts", and thus used when explaining gestalt theory, and further incorrectly applied to systems theory.
So, the final answer is Kurt Koffka.