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I think you have to specify as tier something like

custom-<NUMBER_OF_CPUS>-<AMOUNT_OF_MEMORY>

e.g.

custom-4-4000

which ends up as 4 cpu and 4000 MB memory. I got that from here: https://cloud.google.com/compute/docs/instances/creating-instance-with-custom-machine-type#creating_the_instance and here: https://www.terraform.io/docs/providers/google/r/compute_instance.html#machine_type

Thanks!