There are various synonyms that can be used for the phrase "gains strength" depending on the context. Some of the commonly used synonyms include "improves," "strengthens," "grows," "advances," "develops," "escalates," and "amplifies." These words depict the idea of becoming stronger and progressing towards betterment. For instance, if we talk about personal growth, we can use "improves" or "develops" instead of "gains strength." Similarly, in the context of an athlete, we can use "strengthens" or "advances" instead of "gains strength." In short, using the right synonyms can add variety and depth to the language, making it more expressive and effective.