If that guild holds at least 1 hex on that particular map its name will show up in your guild log. If this guild does not hold any sector on that map, than you can only guess who it was.
As for the guilds name on guild list, if it still exists, you should be able to find it there. However, it id possible that this guild is formed by one man for sole purpose to get some battle points in that particular occasion and than is disbanded. In that case, guild does not exist any more so you will not be able to find it.
On the other hand, it is possible to identify particular player (not the guild) who did battles in GvG if it was someone in your neighborhood, or in your friend list (less likely that you will be attacked by a friend).