All my normal routing goes through my main gateway (lets say on eth1)
However I have another gateway I want to use for specific use (lets say eth2).

I want to be able from the computer side to have some applications going through gateway 2 while the others still use gateway 1. These apps may use different service/ports that are also used by the regular apps. Then a filter via port/protocol on the router side wouldn't work.

I was wondering if the solution wouldn't be to use the socks proxy on the router and to force these specific apps to connect through this socks proxy.
However, I can't figure out on the router side, how to configure the socks proxy to use one gateway and not the other.