The most basic gateway setup is just a Moteino connected to your RaspberryPi, either through the GPIO (GND, VIN, TX, RX) or through the USB (MoteinoUSB).
A step up from that is adding a power button and power control. Below is a photo of what you need. Included is an ATXRaspi board and power button which allows external shutdown/reboots without needing to login to the Pi, but essentially these are not even needed, they are just convenience add-ons. To the right is a sample laser cut acrylic case I’ve built where all this hardware fits nicely:
For reference, here’s another example open-side enclosure I’ve build for a Pi stack with included LCD that displays various messages. This build includes a MightyBoost for power/reboot control (like ATXRaspi) and also battery backup. For more details on how to make an enclosure case like this please see this blog post. This has long served as my own gateway but has now been replaced by a MightyHat driven Pi.