Surprising this isn’t better Google-documented. Here’s how I do it.

Make a list of your existing gems in a text file (run from machine with gems already installed):

sudo dpkg --get-selections | awk '{print $1}' > installedpackages

Copy the file to your new system. Then run:

cat installedpackages | xargs sudo aptitude install -y

It’s run through each line of your gemlist file and run “sudo aptitude install -y” on it.