I solved my problem, thanks to the URL that Richard Rauch 
<rauch@eecs.ukans.edu> sent me.  He suggested that I read Federico Lupi's 
NetBSD Guide, http://www.mclink.it/personal/MG2508/nbsdeng/netbsd.html.  
Chapter 9 gave me the clue that I needed to make my gateway work.

I'm so embarassed to reveal this bit of pilot error: I had set defaultroute 
incorrectly in /etc/rc.d.  I had set it to the address of the non-gateway 
host's ethernet instead of the address of the gateway hosts's ethernet.

Thanks to Mr. Rauch and to Mr. Andrew Gillham <gillham@vaultron.com> for their 
advice.

David "but I read the instructions carefully!" Talmage