Antidote9 is a powerful spell checking program that offers support for both French and English. It has support for most browsers as well as some mail clients, it also supports LibreOffice documents, text files, and *TeX files. While it does not have a Gentoo package/or any explicit Gentoo specific support, the software does work given the right dependencies.
To install Antidote, it is necessary to buy and download the binary installer. The installer will work if all of its dependencies are met. These include (but might not be limited to):