After installing PowerPivot for SharePoint 2010 for the first time, I added a PowerPivot Gallery to one of my existing sites and I was presented with the following error message:
“The referenced file ‘/_layouts/PowerPivot/ReportGalleryView.ascx’ is not allowed on this page.”
After doing some research, I found this blog post. For whatever reason, Microsoft missed a step for the feature activation of the PowerPivot controls that adds the controls to the Safe Controls in the Web.Config file of your web application. You have to do this manually. For each Web.Config file on each server for each web application, you have to add the following Safe Control:
<SafeControl Src="~/_layouts/powerpivot/*" IncludeSubFolders="True" Safe="True" AllowRemoteDesigner="True" SafeAgainstScript="True" />
After adding that to your Safe Controls in each web.config, do an IISRESET and you are all set.