If the user chooses '010.012.000.001' as a gateway IP address (instead of the more commonly used '10.12.0.1', the IP address will be considered as the octal version by route. The installer should transform the IP address into a decimal version before writing the configuration files (as it would be the same behaviour for the gateway and machine's IP address).
see also bug#122038
Fixed on HEAD. Given the trivial workaround, not going to backport to older releases.