Calendars [0] is a simple Event + Attendees model Contacts [1] Re-uses vCard Mail [2] wants RFC 2822 compliant strings Photos [3] have just basic metadata with a URL to fetch pixels Tasks [4] is a simple to-do model
Overall the data model seems like a significant amount of wheel-reinvention. These data models are all just JSON records... they should be JSON-LD with @context pointing to a shared schema, probably defined at schema.org.
[0] https://github.com/google/data-transfer-project/tree/master/...
[1] https://github.com/google/data-transfer-project/blob/master/...
[2] https://github.com/google/data-transfer-project/tree/master/...
[3] https://github.com/google/data-transfer-project/tree/master/...
[4] https://github.com/google/data-transfer-project/tree/master/...