Hi John,
As far as I know there are no PowerShell cmdlets available for managing the web part gallery, but it is possible to write a PowerShell script to access the .Net SharePoint Server Object Model. Here is a blog post that describes how this can be done (there may be better blog posts, this is just one of the first ones that I got on a Google search).
http://troubleshooterms.blogspot.ca/2010/01/deleting-webpart-from-webpart-gallery.html
Now for my big disclaimer:
I have not validated the script on this blog post. Manipulating the gallery in this manner is not something that we recommend or support. Do this at your own risk!
I would definitely validate any scripts that you create in a lab environment before trying it in a production environment.
Good luck.
Bill