DDiego,
well, I have examined the book and what has to say about Gusset plates is around p. 57 that falls short of what now we can do. Enough then, maybe practical, but most would elect doing more now. No mention of the .7 K factor here but somewhere else (not necessarily this book) I think to have read. Since here gusset plates were mostly thoroughly welded this could be coming from considering locally the member compressing as hinged to the gusset and fixed to the other end. The length would be being measured from node to end of gusset for conservatism, or maybe just the exposed part. Think that double angle trusses or channel were predominant. Anyway whilst we find if something a substantiation of this old simplification, let it be stand what said. I check for example Thornton&Kane's contribution to Tamboli's book and find that he takes .5 K factor on length for a corner gusset plate, and this applied to less length than in the assumptions considered above, whatever, so that part of the practice would be conservative per this test. I check the Prontuario Ensidesa and is more or less what in the book by Luis Felipe Rodríguez Martín. Lothers in its classic book also has a table for gusset thickness, as the book, that quoted DIN codes.
What may have me confused in my memory about this data being from this book is that has a procedure at p. 150 for gusset plates used at baseplates to deliver rigidity to the baseplate. There it is specified both length, width of the notional member within the gusset and a 2/3 K factor.
There are out there fantastic books for steel connections, so opportunities to learn are not lacking. Best wishes.