forwarding ssh requests

I'm trying to forward ssh requests from one machine to another from the
internet.

i.e., ssh to my home gateway from work on port 22 does just that (this
works already).
but if i ssh to my home gateway's IP from work, on port 3022 for
example, would actually be logging me into another box behind my
gateway.

I've been googling for iptable rules for this, but i'm not finding one.
Do you guys have any iptable examples or other suggestions?