Even though you may have installed PowerPivot with Sharepoint 2010 and you may have created the root site as a PowerPivot site, any sites you add under that site do not automatically have the PowerPivot Gallery added to them. If you need to add the PowerPivot Gallery to a site, this tutorial will show you how.
Open the main page of a site that the PowerPivot Gallery will be added to with the Sharepoint farm account. Expand "Site Actions" and select "Site Settings".