I found this great post to loop through our sites and get a list of web parts:
Though when I viewed the report, I noticed that many of our Bamboo web parts (e.g. particular AlertPlus and SPGridView and others) were showing up as "Microsoft.SharePoint.WebPartPages.ErrorWebPart".
Turns out that though there are many related assemblies in the GAC, the actual web part assemblies are not in the GAC. So just do a search for the DLLs you'd like, (In my case I dragged the Bamboo.AlertPlus.dll from the C:\inetpub\wwwroot\wss\VirtualDirectories\80\bin to the GAC), closed and reopened PowerShell, re-ran the above script and was on my way with a useful report!