is there are a problem when all gateways are in the same subnet
Gateway 1 192.168.1.1
Gateway 2 192.168.1.2
Gateway 3 192.168.1.3
Yes. That is a problem that houkouonchi is having too. See: http://www.zeroshell.net/eng/forum/viewtopic.php?t=2176
To have that work you would need to be able to specify both the IP address of the gateway and the device to access it over which you can’t do with ZS at the moment.
Could you move the IP subnet assignments around to make them different ranges?