I don't think there is anything really stopping you from standing up a Gentoo system with OpenRC from the systemd installer or vice versa though. It seems really more like a convenience than a requirement to have both installers.
Yes it is possible to change your mind and switch from one to the other after installation, but it's not straightforward.
In the Gentoo kernel make menuconfig there are options for Systemd vs OpenRC - not entirely sure what they do.