to get this 80G you'll need to:
1. Find those newspapers in the cases. if you miss one or two newspapers when you beat the game, you can go to the main menu, cases and then check all your cases and see in which ones you've already found your newspaper. this way you do not have to waste any time finding newspapers you have found before.
2. Beat the game (Moth to a Flame achievement) not much to explain here.
3. Solve the street crimes (The Long Arm of The Law achievement) if you are in a police car you'll often be told about a street crime in the area. when you press A, you'll accept the mission and its location will be shown on your mini map. for every different desk (excluding the patrol desk) there are different street crimes to be completed. what i did was to check out my map after almost every single cutscene in the cases. but i ended up missing 3 street crimes. so i got into free play mode, got in a police car and saw one red indicator for a street crime. i completed this one and was told i completed all street crimes on that desk. i repeated this for every desk until i got all 40 of them done.
4. Find those stupid film reels (Hollywoodland achievement) just some stupid random shit to find and collect. Roosterteeth's achievement hunter Geoff made some amazing guides showing all locations which are shown below. all credit goes to the Achievementhunter office.
5. Discover all special locations (Star Map achievement) whenever you drive past one of these, you instantly "discover" it and it will be added to your map. i would check out this map:
i recommend driving to the locations of the map that haven't been added yet on your map. (NOTE: most of the landmarks will be discovered while going through the story)
6. Drive every vehicle (Auto Fanatic achievement) ouch.. the hard part's come. for this one, there're multiple ways to do this. first, you can check which ones you still need by going to Extras at the main menu. it shows which cars have and haven't been driven yet. there're 5 different types of cars. two of which, police and service cars, i found to be the easiest to find because they all seemed to have some place where they always spawned or spawned during a certain moment in a certain case. then you have 15 special cars, which are all shown on your map whiile progressing through the story.
then you have a lot of 2- and 4door cars. there is a trick for finding these. when at the extras, just go to one of the cars you're still missing, and press A twice, then press B twice and go to cases, Homicide desk, free roam. then go to the parking near your spawn and 8 out of 10 times the car will be there.
the last cars are the 9 sport's cars, for which i have no certain tactic, as i only needed 1 more after i had beaten the game. but i'm told that they often spawn at certain locations, which you can find more info about by googling it.
Good luck! And please if this guide helped you, give it a little thumb up. It'll make me smile :D