Z gauge is 1:220 scale
N gauge is 1:160 scale
HO gauge is 1:87 scale
O gauge is generally 1:48 scale, but can vary from 1:43 to 1:50
G gauge varies between 1:20.5 to 1:32 scale - most equipment is roughly 1:29 scale
Best thing is to look at the Walthers train catalogs - N/Z scale, HO scale, and Big Trains (O, S, & G).