"One by one" is a phrase commonly used to denote a gradual, sequential order of actions or events. However, there are a variety of synonyms that can be used to convey a similar meaning. "Individually," "step by step," and "in succession" all convey a gradual progression towards a goal or outcome. "Sequentially," "serially," and "in a row" are also effective synonyms for "one by one" and often connote a sense of order or organization. "Gradually," "methodically," and "systematically" are further synonyms that emphasize a deliberate and controlled approach to completing tasks or activities. Overall, there are several options for conveying the concept of a gradual, sequential progression that "one by one" implies.