By adding a host, or /32, route to the gateway you should then be able to use it as the default gateway on your main subnet. I was not being serious when I suggested it, as another comment says, it's a pretty stupid idea, and there really isn't any sane usecase for it, except possibly one of those 1am "if it's stupid but it works it's still stupid, but it'll do until morning" type fixes that you shudder to recall years later.