Got two ADSL routers connected to each other and one is also connected to a computer. Both are set to static IP's in the same subnet with their DHCP servers disabled. So the computer can be set to use either of the two routers, depending on the default gateway specified in its NIC settings. Or it can use both routers in alternation by running these commands:

ip route del default

ip route add default scope global nexthop via 192.168.3.1 dev eth1 weight 1 nexthop via 192.168.3.100 dev eth1 weight 1

This works, you end up with twice the bandwidth during downloads.

How can this computer be made into a gateway for a home LAN? Simply setting the LAN NIC to "Shared to other computers" does not work. What else is needed?

You should look into
IP Masquerading
It's been years since i've tried it though