Setting up classless static routes manually in GCE
Solution 1:
udhcpc doesn't request classless static routes by default, you need to instruct it to do so by adding -O 121 or -O staticroutes to the command line. Alternatively DHCP server can be configured to forcibly send the option.
The routes should then be configured by the udhcpc hook script. You can see Gentoo's netifrc udhcpc-hook.sh for an implementation that does this.