You said the original default.aspx worked. Was that in the same web application you are using now? In other words, are you using the same web.config and galleryserverpro.config settings, same support files in the gs directory, etc, so that the *only* difference between working and not working is that one page is stand-alone and the other is inside a master page?
Boy, I don't have any very good ideas, but here are a few troubleshooting ideas:
1. Google the error and see if anything jumps out at you.
2. Download the source code for GSP, compile it and run through the install wizard. Once it is working with the stand-alone default.aspx, add a master page and a new .aspx page that uses the master page. Add the GSP control and see what happens. I can't think of any reason why that wouldn't work.
Roger Martin
Creator and Lead Developer of Gallery Server Pro