To install `htop` on a Gentoo system, you can use the Portage package manager. Here are the steps to do so:

First, make sure your Portage tree is up-to-date by running:

   sudo emerge --sync

You can install `htop` using the following command:

   sudo emerge --ask sys-process/htop

After installation, you can verify that `htop` is installed by running:

   htop

If you encounter any issues or need more detailed information, you can always consult the Gentoo Handbook or the specific package's documentation on the Gentoo Wiki.