How to organize Rails app where each customer has it's own specific gem list?

I'm creating an architecture of the app and trying to find the best one. Few words about it:

• Box solution (each app installed on customer's machine)
• There is a Main app and gems to extend it (private)
• Main app has default list of gem
• Other functionality extending by adding additional gems by request (our gems)

But some customers no need full functionality, so we don't need to include all gems to it's Gemfile.

What is the best way to organize it? What do you think about this way? Maybe you can offer more effective way?