I don't know if anyone has come across this lately, but there is a wonderful overview GG's career and an Obit. in a book called: "The House of Music: Art in an era of institutions" by Samuel Lipman from 1984. I haven't seen this on too many of GG's bibliography references. He is pretty reverential of GG, as he is writing from within the frame of GG's lifetime. Thought I'd pass that along.