Gracepoint is not based on a true story. It is a remake of the British drama "Broadchurch", which was based around the murder of a young boy in a small coastal town in Dorset, England.
The story, though gripping, appears to be completely fictional. The town of Broadchurch is also fictional.
There has been criticism of Gracepoint because the story is exactly the same as Broadchurch. It even has the same lead actor, David Tennant, and the same writer and director. Many people have questioned why they bothered to remake the series at all, particularly as the original was obviously in English, it was not like a lot of recent remakes- such as The Killing and The Tunnel (originally called The Bridge) which were in Danish and Swedish.