They're GO stations with whatever standard service GO provides. The new Finch station is also an excellent example of potential TTC bus service integration with a GO train service.
GO-RER, no matter how good the service is, benefits from strong integration with local transit. We cannot build 20,000 additional parking spaces at every single station but we can run 30 buses per hour, if infrastructure allows for easy transfers and fares encourage it, just as they might at a subway station today. TTC benefits from churn (every seat free'd at a GO station can be filled again before the subway) and GO benefits from a stable base-load.
GO trains should be significantly cheaper to operate and maintain (per customer) than a subway train as well after upgrades are completed.