You can download western saddle gullet templates free from Horsesaddleshop.com. (d/l the patterns, but YOU make the actual templates out of cardboard, plywood, or plastic.) They are SQHB, regular QHB, and FQHB. (Google that b/c I am not explaining.) But SQHB is about like medium gullet for English. Regular QHB might be a wide, and the FQHB is probably going to be XW in most English saddles.
The Wintec gullet gauge is another possibility. But it measures too narrow, say a lot of people. So if it tells you blue MW, then you might actually need a red Wide. Better to go too wide and pad up the saddle a bit than have it be too narrow. If the horse is really wide and table-backed, you might need to go the Wintec Wide series or perhaps the Thorowgood line to get something wide enough.
You can't use a changeable gullet saddle without the metal gullet plate at all, because the metal plate is part of the structure that supports the tree. There are treeless saddles. I don't know anything about them.