It's really not even inspired by Gentoo. Judd was always quite clear that Arch started it's life as Crux with a more automated package manager. The ancestry still shows in the BSD style init, the use of tar.gz as the package format, and the almost identical package build syntax.

The only reason people keep mentioning Gentoo around Arch is because Arch is occasionally concerned with speed and they're both seen as nerd distros.