This is a picture stone that has been re-used as a rune stone a few hundred years after it was carved in the first place. Above the serpents head there seems to be remains of the original design. Point at the picture, to see the back of it, which is the original front of the stone. There you'll see a ship with its crew.
In the background to the right you can see G 370, which has the same shape as this one, and which probably also used to contain a picture from the 5th-7th century. My friend Andrey visited Gotland, and he supplied me with this photo and many others. Thanks !