Gateway problem solved -- everyone read.

Okay.

Fiddling with this and fiddling with that, and using this package and
that package was not as good as good ol' fashioned digging.

Apparently this answer was as elusive to all of you as it was to me, but
fortunately, there was a guru who knew exactly who he was talking about.

Reboot your system, configuring nothing of netfilter or iptables except
exactly what the OS defaults to. Activate your internal and external
networks, and then, speak as the guru did speak, the one magical line
that no one seemed to know about:

iptables -t nat -A POSTROUTING -o <external device> -j MASQUERADE

For me that happened to be:

iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE

And it worked. It freaking worked.

All of my internal computer systems came online, access to every
protocol under the sun, and still I had good protection from the
internet... at least, better than a DLINK router. Connected via the
router, I started LimeWire and became and Ultrapeer. Connected via the
Linux firewall, I started LimeWire as a mere leaf.

That one magic line makes all the difference, and for all the different
"automatic" configuration packages out there, none of them amounted to
what that one line did, which was to actually work.