One of the top options is GPT Workspace. The tool can be integrated directly into Gmail to offer suggestions for improving writing, clarity and authenticity. It also has a number of other features, like grammar checking and content generation, in other Google Workspace apps. With an average rating of 4.9/5 and more than 6 million installations, GPT Workspace is a highly rated and very useful tool.
Another top tool is ChatGPT Writer. It's got more advanced integration with Gmail, with features like grammar checking, rewriting and tone shifting. It works with all languages, and it's available in a variety of pricing tiers, so you can find something that works for your needs and budget. ChatGPT Writer has strong security protections, too, so you can use it without worrying about your data.
If you want a full-featured writing assistant, Grammarly is also worth a look. It offers contextual suggestions and integrates with tools like Gmail. Grammarly's personalized advice and generative AI help you write more effectively and accurately. The service offers a variety of plans to suit different needs, so you can find the right amount of help for your writing.