Where are the changes made in customizer stored?

Which elements aren't working exactly? I'm comparing the two sites you listed above and can only see minimal differences.

the CSS changes haven't taken effect in http://interbridge.com/test/ - i.e. the all-caps site title, the dimensions of the nav bar, etc. from http://leegoldberg.com/ . I thought I was supposed to be able to import those settings, and while I see them in the code of http://interbridge.com/test/ if I view source, I don't see them when I view the site itself.

Ah, remove this line from your child theme CSS:

@import url("../generatepress/style.css");

Thank you!! That worked!

You're welcome :)