Hi Cliff,
It's not documented yet but GAM 2.3 can already modify the sharing settings for both domain (anyone in your Google Apps) and default (any on the Internet). The commands would look like:
gam calendar <calendar email> update read domain
gam calendar <calendar email> update freebusy default
note that there's a bug with GAM 2.3 that prevents these entries from being deleted properly so should be able to turn these on but GAM 2.3 can't turn them off. The next GAM release will fix this.
Interesting side note: While the GCal UI only allows you to give all domain users freebusy and read access to a calendar, the API will let you give all domain users editor and owner rights so:
gam calendar <calendar email> update editor domain
would give all domain users rights to modify the calendar and:
gam calendar <calendar email> update owner domain
would give all domain users ownership of the calendar. Owner would be a bit chaotic though as any user in the domain would then be able to modify the calendar ACLs themselves.
Jay