Start with the Gateway documentation. The company selling it should provide enough information to you that you can discover devices (like the sensor) that have joined the network, and then discover the endpoints, clusters and attributes on those devices.
The ZigBee Alliance has documentation for various layers of their network stack that you'll want to understand if you're going to work with the ZigBee Home Automation profile.
The ZigBee Cluster Library (ZCL) defines attributes and general functions for discovery, reading, writing and reporting on them. You should have a basic understanding of ZCL as a foundation for working with ZigBee.
For the Home Automation Profile, you may need to join the ZigBee Alliance to gain access to the specification. I wasn't able to find it with some quick Google Searches. It will list all of the attributes, their data types, and what values they represent. From that, you can create ZCL Read Attribute Requests to read the values and process the responses.