They are below the buffers, however you may find some GC 180s are set out as 59101/50101.... so that'll be 59 car or 50 car and so the unit number is 180101.
GC's 180s are set out as -
59XXX (Coach B)
56XXX (Coach C)
55XXX (Coach E)
54XXX (Coach F)
50XXX (Coach D)