How to determine eth0 gateway address when it is not the default gateway?
Your mistake is that you're associating the information on the netstat output with the interface rather than the destination. Destination addresses can have associated gateways. When you configure your network, you're associating interfaces and gateways with sets of destination addresses -- so the question you need to be asking is "what's the gateway for destinations in 192.168.98/24?" I'm betting there isn't supposed to be one, that devices in that range are either bridged or directly connected to that ethernet.
Notice that your default route is via ppp0 (the last line is a wildcard route, the mask is all zero so no bits are actually compared, so packets for those addresses are sent via ppp0). So packets for any destination not in 127.xx.xx.xx or 192.168.98.xx will eventually match the wildcard mask and be sent over ppp0. Now, because it's a point-to-point protocol, every packet sent on that interface goes to the only device directly addressable on that link, the one at the other end of it. There's no need to specify a gateway for it because every packet is sent over that interface is sent via that device, whether its Internet destination is that device or not.
Now: if you have a router accessible over eth0 that you use when you can, that router almost certainly has an IP address in the 192.168.98.xx range, at least as likely as not 192.168.98.1, and when that router's powered up and actually connected to the Internet you want your default route, the one for everything not directly connected to a local network, to be default via 192.168.98.1
You might have the newer (that's a relative term, it's been around for a very long time now) ip command in use, try ip route and if that gives you some output try ip route help. Otherwise, try route help.
So, your eth0 interface simply doesn't have a gateway address. There might be another device or several devices connected to that ethernet willing to serve as a gateway. You say your network's got one that responds on the (purely conventional) .1. No law says it has to have that address, and no law says it has to advertise its presence. There are some common ways, the most common of them being for boxes running routing software to also run dhcp servers and tell anyone who speaks dhcp about themselves that way. MolbOrg's answer gives a pretty typical flavor of the setup, but anything anyone can suggest at this point is only going to be more or less likely to match your particular network.