Setting nic address and default gateway as the same ip

[daftunderchem]

Hi everybody,
i have recently added a new wan (the third) to my network, but the ISP wants some strange configuration.
The situation is:
WAN subnet is 192.168.200.0/24
192.168.200.1 is the ISP router addres
192.168.200.2-17 are 16 ips tha my ISP map 1:1 to 16 public ips.
i've got a new wan interface on pfsense whilch have 192.168.200.2 address
The problem is that my ISP wants that every machine that take one of theese 1:1 mapped addresses
is set up with a default gateway which is the same ip address as the nic address, otherwise they don't let me access the internet.
I want to assign theese 16 ip to machines that are on a DMZ (192.168.100.0/24) so i've got 16 virtual ip and 16 corresponding nat 1:1 on my pfsense box but i don't know how to asign them a different default gateway as my provider wants, because they all have set pfsense DMZ interface address as default gateway.

i've heard that this configuration was largely used for dial-up connections
but i don't know how to adapt it to a pfsense managed network
What can i do?
thanks everybody for help