Grey most of the time means that it is downloaded before or is currently in your download queue.
However, gtk-gnutella uses the SHA1 of the file to determine wether it was allready downloaded. So one file named differently will in either case be seen as downloaded/downloading. This is why you might see names in grey even when you haven't downloaded that file name before.
In ~/.gtk-gnutella/done.* are the files previously downloaded.

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