Hey Joe,
You will not be able to transfer that custom field into Google Calendar sync, you can add an Action to transfer whatever that is in the custom location Field into the Activity Comments which do show up in a Google Calendar sync.
If you have location data in the Google Calendar event (the 'Where' field in the Calendar Event) it will sync over into Method within the Comments section.
E.g. "Location: McDonald's, 160 Spadina Avenue, Toronto, ON M5T 2C2, Canada"
We create feature request tickets internally. I went ahead and created one to transfer the Google event location to a custom location field rather than the Comments section. I'll follow up with what I hear from the developers in regards to API restrictions.
- Mortaza