You translate in the same manner you would any other CB language file. It uses the same structure as CBSubs (except designed with new Tag support). If you have an error it's likely due to an unescaped quote. Example below would cause an error

'STRING' => 'STRING'S'

The above has an unescaped single quote. The below would be correct.

'STRING' => 'STRING\'S'

Yes, the above is correct. You translate GJ like you'd translate anything else, simply edit the strings in the language plugin.

It's recommended to translate the English plugin, edit so it'll install as your own language, test install is working, then distribute. I do not recommend distributing edited English plugins as you should always create a new plugin associated with the language GJ is translated to. Files do not need renamed, you simply need to edit the XML file. Please see below example from English to Russian.

IN: cbgroupjive_language.xml
ON: Line 3
FROM:TO:
Once done, package and install. When your Joomla language switches to Russian, so should GJs in the above example.