I am not sure why you are getting that - haven't come across it before. I don't do anything with signing assemblies.
Just a guess here - do you need to unblock the ZIP file before you extract the contents? I don't think that's it but I'll throw it out there...
Have you done anything to alter the source code after you downloaded it? Is there anything non-standard about your setup? Can you try it on another PC?
If all else fails, I would start googling the error message.
Roger Martin
Creator and Lead Developer of Gallery Server Pro