If you get this message
Microsoft.AnalysisServices.SPAddin.ReportGallery.ReportGalleryView when you try to create a PowerPivot Gallery here’s what you should do.
Deploy powerpivotwebapplicationsolution.wsp to your web app
To do this you go into the central admin, manage farm solutions. You can find it under System settings. There you can deploy the solution to your web app. (it is probably only deployed to the central admin web app)