Hi there,
I'm really sorry to hear about this. I'm not sure what the problem is, as GOG makes the installers. I will follow up with them about a resolution, and they may contact you for more info if you can help them. But in the meantime if you want to send me your proof of purchase at arcengames at gmail dot com, then I can send you our own drm-free package for the expansion, letting you get that in place while the issue with the GOG installer is sorted out.
Thanks for your support and understanding!