I just platinumed my copy of Steins;Gate on PS3 and since I had real trouble working out how to get the 100% CG trophy I thought I'd make a note here. I have this YouTube vid to thank for it.
Go to the Stardust Sky chapter, the one that leads to the Mayuri ending where you haven't got any of the true ending flags. Once you've shifted back to the Alpha wordline, you'll get a new email with the last CG. I'm assuming this only works after you've collected all the other CGs and completely licked the game otherwise, though I don't know. Anyway, 100% CG for you.