The handling of gateways differs between IPv4 and IPv6 in the template snippet `preseed_network_setup`.
For IPv4, it writes a gateway entry only for the primary interface.
For IPv6, it writes a gateway for all interfaces.

However the test in the template is also wrong (in Ruby an empty string is true) and for IPv6 it writes an empty address, which is a parse error.

<% if host_subnet6.gateway -%>
    gateway <%= host_subnet6.gateway %>
<% end -%>

IPv4 and IPv6 should be configured equivalent by Foreman, aka gateway only on the primary network until it can setup source routing required for multiple default routes.