systemd has no scripts to start networking on its own. For systems who don't use some sort of network manager(i.e servers, workstations, virtualized instances) this becomes a very big problem. Reproducible: Always
That's not really systemd's job, and upstream does not ship anything of that nature. Install a network manager. Personally, I use net-misc/netctl, which integrates with systemd quite nicely. It's really just a set of bash scripts, so the system requirements are quite low. I believe some of the Gentoo devs are working on decoupling the network config system from openrc to be able to use it with systemd.
Fedora does something similar to that with initscripts(http://koji.fedoraproject.org/koji/buildinfo?buildID=411153)
File a request upstream or file a specific Gentoo inclusion request for something we don't have packaged.