Probably mentioned on here at some point already, but all three words, hit, the and gas are too short for the search function so it won't show up.
You basically can't 100% the game if you finished the main story line, right now.
EDIT: nvm, while I still never did Hit the Gas, nor is it shown in my completed list, the final thing was shooting the YES sign, which funnily enough, I did try before I liberated that region, but nothing happened...