It is difficult to ascertain if he played with nails (perhaps his fingertips were hardened by years of playing) but his sound, clearly on a fine instrument, is often very beautiful, particularly as recorded by DGG Arkiv. Unfortunately, there is no biographical information available apart from the brief Wikipedia pieces and a little more (with photograph) at www.bach-cantatas.com/Bio/Gerwig-Walter.htm
. The LP/EP recordings (including the 3-LP set 'Lautenmusik der Renaissance aus Deutschland, Italien, Frankreich und England' - Harmonia Mundi 1C 149-99 605/07) give no information anout him. You may have ascertained that the South American River site has one CD by him, of music by Bach (Mucicaphon B00004SZWZ) price approx. $5.00. Some reviews by 'Gramophone' and other critics can be found by Googling - one mentions that Liona Boyd was a great fan of Walter (good for you, Liona).