To Gold Star any song, you must hit 100% of the notes. It does not have to be a full combo (You're allowed to overstrum) as long as you HIT EVERY SINGLE NOTE.
You must administer this to 20 songs in Expert.
'Easy' songs to start with, are Mississippi Queen, Radio Song, Avalancha, and Closer.
I guess its worth mentioning there's a glitch you can use to unlock this achievement, but if you're going to cheat the achievement, then doesn't that ultimately take the actual feeling of achievement out of it?
(No, if you want that glitch, search elsewhere.)