The phrase "gaining upon" implies progress, advancement, and catching up to something. So, the antonyms to this term would be to retreat, fall behind, or regress. Conversely, the opposite of "gaining upon" is to lose ground, withdraw, or backtrack. While "gaining upon" signifies positive growth, its antonyms denote negative outcomes like stagnancy or losing ground. Therefore, it is imperative to understand the context of "gaining upon" to appreciate its antonyms. In conclusion, antonyms for "gaining upon" are crucial to express the opposite of moving forward, either in a tangible or intangible sense.