The pre-ordered retail version should come with the Imperial Edition. The included GOG key is for the standard release, so presumably either the Steam install includes the Imperial Edition or (more likely) there may be a key or download listed in your Larian Vault account. If not, and you pre-ordered the game, email support@larian.com.
There were some issues with switching the Steam pre-orders over to the Imperial Edition, but I don't recall any reports of the Planet Earth map disappearing. In your Steam library, if you right click on Dragon Commander, select Properties and switch to the DLC tab, you can check if the Imperial Edition is listed there.