Why Gemini text sounds machine-made
Gemini likes to answer in a tidy structure: a short introduction, bullet points, bold keywords and a summary at the end. In a chat that helps, but in an essay or an e-mail it looks like the output of a tool and not a text by a person.
Inside the paragraphs the same applies as with other models: sentences of similar length and generic turns of phrase. It shows even when the text was written directly in Gmail or Google Docs and feels like your own.