Gravia sometimes called Kravia is not in the same island group as Matthraki, Othoni and Erikousa which are further out and further north.
Gravia and the 2 smaller islands Sikia and the end one Yineka as you know are just off the coast of Arillas.
The resort report on the Agni.gr web-site says the writerwas told there are lovely sandy beaches on the othe side of Gravia. Having been around in a boat I certainly didn't see any .... just big rocks and cliffs into deep water.
A snorkeling friend fishes there in the deep water but said you can get on land with care in places. It is just a lump of rock though so not much walking. The sea was too choppy for us to risk damaging the boat by landing.
Don't know about any history but have read that legends state it is a ship of Odysseus turned to stone (karavi is a word for boat).
Go to
http://www.afionas.gr for more about it. Click on the Union Jack and then choose History.
Dennis