That is kind of odd. I did an internet search and found two possible causes:
1. A closing comment tag (-->) not accompanied by an opening tag. Check your web.config for any issues with comments.
2. Some kind of encoding issue. One person reported that he fixed the problem by opening web.config and saving it in Notepad.
If you continue to have the problem, I would replace your web.config file with the one that ships with GSP. You'll have to manually update the connection string and tell the membership, profile, and role provider sections to use SQL Server instead of SQLite.
Roger Martin
Creator and Lead Developer of Gallery Server Pro