We've been up and running in Marketo since March of 2015.
When you create a Smart List Theres a filter you can select that's called 'Not Opened Email'.
If you set the name to 'is Any' remove the date constraint that should bring them up.
Please be aware that Gmail automatically caches images and will therefor show as 'opened' even though they might never have been really opened.
Very interesting, thanks for the help.
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