Gamer has been removed
Ok I just have to put this here because it would be tedius to comment to every solution already here.
FIRST OF ALL, READ ALL OF THE OTHER SOLUTIONS, AND WATCH THE VIDEOS.
This is not a solution, but an EXTREMELY IMPORTANT note to remember.
The mission, "Frosting on the Cake", where you drive a limo and then use sticky bombs out of the limo later, WILL NOT APPEAR ON YOUR STATS PAGE (in the "General" tab). If you skim your stats and see all 80+% or 100%, and you DID NOT unlock your achievement, it's because you need to improve your score for "Frosting on the Cake".
If you look in the "Player" tab, you WILL see the score for "Frosting on the Cake" in there.
DO NOT SOLELY RELY ON THE GENERAL STATS FOR THE GAME.
I went through two entire playthroughs and over a month of headache and no help from Rockstar support before realizing this.
Good luck getting your Gold Star!