Google Calendar for iOS is launched; download it for free from the App Store
Based on the reviews from those who have already downloaded and installed the app, it appears that there is no support for landscape mode. No doubt that Google will add this capability in the near future via an update. Meanwhile, there are a number of iOS users who have been patiently waiting for this app to appear.
Google Calendar joins other Google apps for iOS including YouTube, Google, Google Chrome Browser, Gmail and YouTube Kids.
source: GoogleCalendar (iOS)