FileMaker files completely unlocked that lets you connect your Google Calendars to FileMaker without having trouble with API; It uses Google API to send event to Google Calendar and to receive events from it (you'll need to activate you token/access code in Google developer console).
The interface is intended for developers, not for users; the file is a connector to upload to a Filemaker server. A set of instructions will guide you to connect your table(s) with Google Calendar.
There are few script you can call from your database (even with PSoS) passing some parameters.
The primary scope is to SEND events to gCal. You can use many calendars and connect every table you want with a simple "edit" relationship.
Secondly you can download the events of a calendar (or all calendars); The App will download all events in a dedicate table: you can connect this table to you database or import events from it.