It can be made to work, just really really stupid.

Ultimately the 'gateway' is really a mac address, and the IP is just the thing to ARP to get the mac address. And you can do some gnarly things to pull off things like off-subnet gateway addresses.

You should never ever do it, but you can if you hate yourself enough.

Becomes easier in IPv6 to do these shenigans too.

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.