Yup! Opta was initially designed to split between one "environment" manifest holding all the networking & cluster setup and many different "services" built on top of the environment. However, it is possible to have an environment and service in a single yaml, and to keep things simple that's what is being done in many places in this repo. You can still create services pointing to the lone yaml here as the "environment" (even though it deploys what can be considered a service as well), but we'd still advise you to break the current yaml to an environment and service manifest when you feel ready just to keep things clean.
Opta is cloud-agnostic and fully supports GCP as well! Chances are it is missing due to brevity and our limited resources, but if you'd like us to show an example in GCP then please let us know in our slack.