Old thread I know, but...
Be aware that in IP networking a "default gateway" and "default route" is not quite the same thing.
Client devices o a LAN have a "default gateway" ie where to send any IP traffic we don't otherwise know what to do with which in the SOHO use case is nearly always your router's internal IP address - eg 192.168.X.Y.
Routers have more complex routing decisions to make than client devices (though for the SOHO use case, not much more complex). They consult a table that describes where to send traffic based on it's destination (and sometime source) IP address. As such, routers do not have a "default gateway" but they may (often) have "catch all" route at the bottom of their routing table, which again for the simple SOHO use case is will be most likely somewhere up at your ISP. And may well be different for different users. For a SOHO network, the default route is almost certainly acquired (using DHCP) when your router connects to your ISP. (For "proper" big boys routers that I manage professionally, sometimes the "default route" might be something we "hard code " or might be to simply drop any traffic we cannot figure out what to do with! Or some equally obscure torture.)
Due to public IP (V4) address exhaustion, some ISP's use "private" IP address ranges upstream from the SOHO users (e.g. 172.16-31.X.Y or 10.somethings) before NAT'ing the traffic out onto the public Internet proper.
None of this is anything to "worry" about, I would leave it well alone unless it's not working in which case you'd be best to contact your ISP rather than try to "fix" it yourself unless you really know what you are doing. To mis-quote Gene Kranz "don't potentially make things worse by guessing."
Incidentally, I tend to go with the ISP's provided DNS servers - they will be "closer" (in terms of network hops) to you than third party (which by definition is "outside" your ISP's network) and should be a little quicker to respond - not that I think it will be easy to measure, let alone "notice" in normal use, but I can see the arguments for those that want to use an alternate.
