Gentoo is described as 'Linux is a versatile, fast, highly configurable and completely free Linux and FreeBSD distribution geared towards developers and network professionals' and is a very popular Linux Distro in the os & utilities category. There are more than 100 alternatives to Gentoo for a variety of platforms, including Linux, Self-Hosted, Windows, BSD and Mac apps. The best Gentoo alternative is Linux Mint, which is both free and Open Source. Other great apps like Gentoo are Ubuntu, Debian, Arch Linux and Fedora.
As cakedy said, Gentoo is geared toward experienced Linux users. Also, Ubuntu includes a lot of software from the start, whereas with Gentoo, you would install everything manually and customize it to your liking.