There are two city's south east of the map, if you go to the one on the south of the other, there will be one road longer than the rest.

1) Find a ambulance
2) Set up at the top of the long road (towards the water)
3) Drive down the road into the water at the end
4) You should see a black and white screen and the collecting achievements will start popping up
5) Wait till they're all done.
6) Restart your Xbox 360 and admire the easy GamerScore :)

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