ARP packets won't travel across different subnets. So no. However are you sure it's 192.168.2.1/24 and not 192.168.1.1/23?
Even so, it depends what you want to do ARP cache poisoning on. You can poison route the proxy and the client. It doesn't HAVE to be the gateway.
Furthermore, check this out: http://www.oxid.it/downloads/apr-intro.swf
It has almost everything you need to know