FileMaker files completely unlocked that let you connect your Google Calendars to FileMaker without having trouble with API; It uses Google API (you'll need to activate you token/access code in Google developer console) to send event to Google Calendar and to receive events from it.
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 (passing parameters) from your database (even with PSoS)
The primary scope is to SEND events to gCal. You can use many calendars and color and connect every table you want with a simple "edit" relationship.
Secondly you can download thee event of a calendar (or all calendars); The App will download all events in a dedicate d table: you can connect it directly to you database or import events from it.