I, too, don't understand the vicious loathing of Grey Flannel. I can only attribute it to the Basenotes/YouTube/Marc-Tim-Brandon-says-it-sucks-therefore-so-must-I hivemind that casts a pall over the entire fragrance review community, but that's me. It's a lovely, crisp, well-turned-out scent that clearly hails from another era, but doesn't smell at all musty or "old-fashioned".
IIRC, one of the more coked-up asshats on YouTube, MyMickers, issued a blind-sniff challenge of Grey Flannel to his subscribers, and several responses flooded in, all unanimously negative. One of them compared the smell of GF to "an old lady's crotch", which says more about the reviewer than the frag, if you ask me.