DHCP client support for "Classless Static Route" option
RFC 3442 describes a DHCP option used to provide a static route entry to the client. This option is quite flexible, and allows for both next-hop style routes (where a prefix is directed through a locally accessible gateway) and for local routes (where an off-subnet host is described as being directly accessible).
Google Compute Engine (GCE) uses a somewhat eccentric network configuration, which it describes to the guest using this DHCP option. In particular, the DHCP server provides a 32 bit subnet mask; i.e., there are no on-subnet hosts. The DHCP server then provides a classless route option for an off-subnet gateway IP, and a default gateway entry using that off-subnet IP.