...the un-ending desire for more/better/newer stuff, often leading to financial irresponsibility and/or irrational justifications.
I think it was "guitar" when the expression was first coined, but it's been expanded to "gear" in general usage. Why should the guitarists have all the fun?
I play with a lot of sax players. A few might have two of their main axe (e.g. tenor saxes) but I'd say the majority only have one. Of course, they also have to have an alto sax, a bari sax, a soprano sax, a clarinet, and a flute...so they have their own version of G.A.S.