No need for a gem for this one.
Simple Migration and Seeds file for adding Countries to your Rails app.
Run the migration or use the rails generator
rails g model Country name:string:index offical_name:string sovereignty:string iso_alpha_two:string:index iso_alpha_three:string:index iso_numeric:integer:index subdivision_codes:string tld:string
Copy over your seeds, seed them in your usual way, and viola!
That's it!
Here is a gist with a rake task that will allow you to import these seeds from the file directly:
https://gist.github.com/CromonMS/2edc3ccc254ef9baa34d61252530a566
If there are changes / updates to the country list, you can run the parser to update the seeds file.
ruby parse_wiki.rb
When you run the parser there are a few countries that are broken and need manually fixing, mostly due to punctuation and quotations etc.
I do not have time to work out the problem, more than happy to take a pull request that deals with the errors that happen.