The words gradience, gradients sound the same but have different meanings and spellings. Why do gradience, gradients sound the same even though they are completely different words?
The answer is simple: gradience, gradients are homophones of the English language.
The absence of a clear boundary, or the presence of a continuous spectrum of meaning, between two words or categories
Plural form of gradient.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").