Just to illustrate, I asked both about a browser automation script this morning. Claude used Selenium. Gemini used Playwright.
I think the main reasons Gemini is much better are:
1. It gets my whole code base as context. Claude can't take that many tokens. I also include documentation for newer versions of libraries (e.g. Svelte 5) that the LLM is not so familiar with.
2. Gemini has a more recent knowledge cutoff.
3. Gemini 2.5 Pro is a thinking model.
4. It's free to use through the web UI.