Which one only for email and short messages?

Both work. ChatGPT wins if you already have a Custom GPT with your tone. Gemini wins if you draft from Gmail or attach a captured thread and want subject + body in a fixed format.

What about planning the week or a meeting?

Always ask for table or checklist output. Gemini fits when calendar and docs live in Google; ChatGPT if you prefer many variants and saving the brief in a Project.

Does one fully replace the other?

You do not have to choose. Many teams pick one primary tool by ecosystem (Google vs OpenAI) and open the other on free to contrast a delicate paragraph or a fast draft.

Which one for source-based research?

For news or open topics, use either and open the sources. If the deliverable is citations only from PDFs you uploaded, use NotebookLM instead of forcing Gemini or ChatGPT.

Gemini vs ChatGPT for daily work

Two generalists for the desk: one fits Google, attachments and Workspace; the other fits GPTs, Projects and the wider ecosystem. Email, docs, light research and planning — no hype.

	Gemini	ChatGPT
Best for	Email, Docs, Drive, screenshots and plans with Google context	Quick drafts, GPTs, Projects and a generalist outside Google
Daily context	Multimodal attachments and Workspace extensions on plans that include them	Projects and long threads; paste what you need or start blank
Repeatable flows	Gems and saved instructions; smaller marketplace than GPTs	Custom GPTs, Projects and community templates that are very mature
Light research	Summary and synthesis with search per plan; cross-check dates and sources	Built-in web search; good for orientation, not for citing without opening
Planning and short docs	Tables, checklists and notes when you ask for explicit format	Very agile at tone variants and first versions of text
Multimodal	Strong on image and PDF in the same work chat	Images, voice and file analysis per plan and model
Entry plan	Free with limits + Google AI Pro / Workspace with Gemini	Free + Plus / Team
Main risk	Assuming Workspace «already knows» a doc you did not attach or share	Shipping the first draft without flagging uncertain figures or facts

Verdict

Pick Gemini if your day runs through Gmail, Drive, Docs or Calendar and you want screenshots, PDFs or Google context without a separate project setup: email drafts, meeting summaries, weekly tables and Gems for a repeatable style fit well. Pick ChatGPT if your flow lives in reusable briefs, Custom GPTs, multi-thread Projects or you want the generalist with search, voice and images in one place even outside Google. For light open-web research, either works if you open the links; for a closed corpus with mandatory citations, neither replaces NotebookLM. A common combo: outline and variants in ChatGPT, paste and polish in Gemini when the deliverable lands in Docs — or Gemini alone if you already pay for Workspace with Gemini.