Have you flashed the Xposed .zip before installing the Xposed Installer app?
The app in FP Open OS is the standard android calendar. In the app it says “about Google Calendar” instead of “about Calendar”, but if the app is called “Google Calendar” in the app launcher then you didn’t succeed in installing FP Open OS.
While this may be true technically. (Some DNS thing still uses Google afaik, whatever that is), but:
This is not true for FP Open OS. The standard android apps that are pre-installed are created by the Android Open Source Project (AOSP) which is financed by Google, but the apps don’t need Google (in some cases, like the calendar it’s not too easy to use without a Google Account, but you’ll find ways to do it here on the forum).
The topic @Lidwien linked to above is a great start. (You can “summarize” it to see the most-liked and -replied-to posts). Also in the wiki linked in that topic the last point is the guide to installing Xposed on the FP2.