[Fa-koff]

I'm not sure of how you're trying to 'tunnel' nor why, but most times you can simply use ssh to dynamically forward traffic through your sshd on your server.
The following sets up a SOCKS5 proxy on localhost using port 8080:

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ssh -C -D 8080 -fN [email protected]

The "-C" means compress the data; the "-D" means "Dynamic Forwarding; "8080" = local port; the "-fN" means to fork the process to the background, and Not to execute commands on the remote host.

Then setup your application to use the ssh connection, which depends on which app you're trying to use.

Example for Chrome/Chromium:

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chrome --proxy-server="socks://localhost:8080"

For Firefox go to Preferences > Advanced > Network > Settings;
Choose "Manual Proxy..." type "localhost" in the "SOCKS Host" field and check "SOCKS v5" and set the port to the same one you used in the first command - in this case it's "8080."
Spoiler (Click to View)
Then go to a new tab (Ctrl + T) and type:

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about:config

in the URL field.
Click through the nag screen, and type the following into the "Search" field:

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remote

This should return something about "blah-blah-remote.dns..." where you right-click on the last field to "Toggle" from "False" to "True." (this sends your DNS requests through the SSH connection)
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Close out of that, and open some site that tells you what your IP address is to see if you're using the SSH "tunnel."

Spoiler (Click to View)[img]http://moco-sux.net63.net/dynsig2.jpg[/img]

[Fa-koff]

Setup your sshd_config to only allow connections from specified users from their respective IP addresses using public keys, deny all extraneous connections.

Then setup your firewall to drop all connections except those you want to allow.

[Fa-koff]

Why not?
You should already have the necessary information, you've got the computer... Use it.

[Fa-koff]

Yes, you'll need to add the allowed IP addresses, and you already have that information in /var/log/auth.log

Just cat | grep through the file for the username/IP address combos and > to a text file from where you can use this list to populate your sshd_config and your firewall rule set.

I suggest that you do some more reading regarding all of the steps involved in order to gain understanding.

[UnspokenOne]

In regards to your GRE problem, your kernel probably doesn't have support for loadable modules (and/or you don't have the GRE module)... you would need to rebuild your kernel.