How do I restore a Deja Dup backup (done in Ubuntu) in Mountain Lion? I have pretty much started from scratch on my new installation because I couldn't figure out how to restore the files I backed up on my Linux installation with Deja Dup. I have GPG Keychain access installed, so what do I need to do to restore the files I had and look at them and see what I want to keep?

I really appreciate any answer you can give :)
Deja dup uses duplicity for storage. You may want to use duplicity in the command line, which is available for macOS.

Install it with Homebrew like this (install Homebrew first if you don't have it yet):

brew install duplicity

Then restore with something like:

duplicity --file-to-restore the/file/you/want/to/restore file:///backup/location /where/you/want/to/restore

Not sure if this will help OP, but could help other people.