In Phex we mostly used the specs as worked out in the_gdf:
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http://groups.yahoo.com/group/the_gdf/files/
(you need a yahoo login to get there - grown structures...)
Also the current specs are mostly at the site maintained by LimeWire:
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GDF - LimeWire
(the ones from gtk-gnutella are quite good, too, though)
Some more specs can be found in the phex code repository:
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SourceForge.net Repository - [phex] Index of /phex/trunk/docs/Gnutella