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I tried this using the version 2 syntax on an openSUSE VM and it worked. I think it should also work in Ubuntu:

network:
  version: 2
  ethernets:
    enp1s0:
      dhcp4: true
      gateway4: 192.168.1.88
      nameservers:
        addresses:
          - 192.168.1.88

That'll change the default GW using the same netmask as gotten from DHCP for the interface. If what you want is setting other static routes this should work:

routes:
        - to:  192.168.1.0/24
          via: 192.168.1.88

Appreciated! 😄 @quick-sandwich-76600