For example, somebody would like to install lora gateway on raspberry pi ( or pi zero ) in docs there is link here
As you describe, building a gateway from scratch can be a lot fo work. This is the reason why we point to a pre-build image that you can flash on a SD Card and includes either the Packet Forwarder + LoRa Gateway Bridge or Packet Forwarder + LoRa Gateway Bridge + LoRa Server + LoRa Server, including all the dependencies.
what if I have already installed image and just want the gateway software where should I get it
You can use one of these binaries: https://artifacts.loraserver.io/downloads/lora-gateway-bridge/
We provide binaries (and Debian packages) for armv5, armv6 and armv7. I believe for the Raspberry Pi Zero you could use the armv6 (not tested).
there is no mention in documentation how to make it work ( also if you use custom shield
Again, that is why we provide images These images come with a
gateway-config utility to setup various shields and configure the reset pin.
Again, if you think there is documentation that could be improved, then I welcome you to contribute to the documentation