I never really got “who is John Galt?”. Everybody in the book seemed to know the phrase, but it was not clear (unless I missed it or forgot) how it ever started. It seems it should have started appearing as graffiti like “Kilroy was here” or on leaflets or something; but everybody in the book seemed to know the phrase with no reason for knowing it. I think the book just said it was a well-known catch-phrase that no one knew the origin of. It seemed to be used when someone was meaning to say “who knows”?
“Who is John Galt?” is a cry of frustration at the inability to change anything. Today we hear the phrase, “It is what it is.” That’s our modern equivalent of it.