I know in the Google Apps Marketplace SDK you can specify a Gmail Contextual Gadget in the COB Extension section however there doesn't seem to be anyway to have a Google Apps Marketplace app install Calendar Gadgets automatically.
Barring that, is it possible for the Google Apps admin to install the Calendar Gadget for all the users on the Google Domain directly?
It doesn't seem like either of these is possible, is this correct? If so it's really disappointing.
The only way to install a calendar gadget is for each user to click on the install URL for the gadget. They need to accept the gadget and then it is installed and available.
You could look to use a chrome extension instead. Although this has its own installation challenges for a domain.