I'm looking for a solution to create my own LoRaWAN gateway that can also be the server.
I would like to get data from any classic LoRaWAN sensor to my own gateway, decrypt and store them locally without an Internet connection or the TTN network.
Is it possible, and is there a way to begin?
As the LoRaWAN protocol is quite extensive, it seems not reasonable to implement the gateway as well as the server on your own. Instead, you could take open source implementations of all necessary components and integrate them on one host.
For the gateway, you will need a low-level interface (SPI) to the LoRaWAN gateway/concentrator chip (Semtech SX130x) of your hardware. This is possible on a Raspberry Pi, for example. It depends on you specific gateway module, of course.
For the other infrastructure you could resort to either The Things Stack or Chirpstack, both of which can be installed on a decent Linux box (including Raspberry Pi). Docker images are available, if I remember correctly. Communication between gateway software and LoRaWAN server is via IP, so there isn't any problem doing this locally. You can even add the backend software on the same host, i.e., Node-RED, InfluxDB, Grafana, etc., as far as hardware resources (CPU and RAM) will suffice.
PS: I just discovered that—of course—exactly all this has already been done: Balena Standalone LoRaWAN gateway.