You should just set the gateway on each machine as themselves. Since they are on the same subnet they don't need a gateway.
how exactly do you want to share files? you can do so by setting up an ftp, ssh, samba, nfs server on one(or both) and do it that way. I suppose the easiest is via ssh because it's probably already installed on both and encrypted(not that anyone is going to snoop on you single cable network).
use sftp or scp to copy files from one place to the other.
scp filename username@SSH_Server:/path/to/dir
* use the -r switch to do recursive(all directory)
Also did you check your firewall? that could be blocking traffic. If you want to do set one up as a gateway, I would just look for some software that does it for you. I'm sorry but, I can't give you a particular package name but, there plenty out there.