Gunnar Guillermo Nielsen (Spanish pronunciation: [ɡũn.ˈnaɾ ɣi.ˈʎɛɾ.mo ˈnjɛl.sɛ̃n]; born 12 October 1983 in Posadas, Misiones) is an Argentine football player of Danish descent, currently playing for Hillerød GI. He previously represented FC Fredericia and Kolding FC.