Handbook:Parts/Blocks/Systemd

Optional: Using systemd
The remainder of the Gentoo Handbook focuses on OpenRC as the default init support system. If systemd is wanted instead, or you are planning to use Gnome 3.8 and later (which requires systemd), please consult the systemd page as it elaborates on the different configuration settings and methods.

The Gentoo handbook can then be followed with that page in mind.