Getting Gold Stars needs about 30% more points than you need to get a five-star on the song. You need to pass about 90% of the moves to get gold stars and get a handful of Flawless moves.
It doesn't matter what song or difficulty you do this on. "Poker Face" on Easy is basically arm swinging and some left-to-right steps.
Important Note:
It is better to get this achievement LAST, as there is currently a bug which prevents you from getting other 5-star achievements such as
Look Ma, No Flashcards!EDIT:
On Hard, it is an absolute necessity that you Flawless at least 80% of the moves if you want a chance at gold star, even then you may need to pass them all. Always try for this on Easy.