Originally posted by SwissGambit
Database = collection of games. So yes, if they find a GM game with the same position, they can copy the GM's moves. As for gaining an advantage - it depends on whether they understand the ideas behind the GM moves well enough to know what to do if the opponent diverges from the game line.
I usually don't understand what's going on in the openings, especially at the moment, I certainly don't understand the ideas behind GM moves because my current openings are very unfamiliar to me.
I used to have nice and small 1.c4 repertuare, where now I have switched to 1.d4 and the material is like 50x larger, I used to play narrow lines of french and now I play the petrov against 1.e4, they are two completely different worlds.
I even have to have something against that stupid KG, it's just a nuts opening. Anyways, I played two KG games after switching as black, and I was absolutely clueless, but got into very favorable positions (probably winning) with the help of my databases. Databases are especially helpful if your opponent goes for a little dubious but complicated openings, they may be very hard to refute on your own, but with databases, it's not such a big problem as it would be OTB.
So when I'm out of book, I'm like a fish out of water. But I still believe databases give me a serious advantage if my opponent doesn't use any, with everything else being equal.