In many cases very similar plants is not the same. Firstly decide what Gnaphalium polycephalum you means? According to http://www.catalogueoflife.org/ Gnaphalium polycephalum Michx. is provisionally accepted name, but Gnaphalium polycephalum Willd. ex Spreng. is a synonim to Helichrysum nodiflorum DC. (provisionally accepted name) (http://www.catalogueoflife.org/col/search/scientific/genus/gnaphalium/species/polycephalum/match/1). Helichrysum italicum (Roth) G.Don (http://www.catalogueoflife.org/col/details/species/id/14579784) is also provisionally accepted name. If names accepted after revision it means that it is two separate valid species. I recommend you to find the description of Asteraceae genus in local flora. Maybe you find some differences here. Another good way to solve this problem - to find the first descriptions of these species. Yuo can find all necessary bibliographic references on http://www.ipni.org/. You may to type name of the plant on this page http://www.ipni.org/ipni/plantnamesearchpage.do, to get the reference clique on author of every taxon.