The google sheet must be public and must have have these 3 columns:

1. Position: String for timeline row that record belongs to
2. Name: A label for that row on the timeline
3. Start: A date for the beginning of that row in timeline
4. End: a date for the end of that row in timeline

To make it public you need to use File -> Publish to the internet
An example could be found here https://docs.google.com/spreadsheets/d/19AoR5siwI-cbrlJN47cZV7nhoZ9mO76XBUAWtO5Xb8A/edit#gid=0