Interpreting CAN bus data from OBD port
The CAN bus is not like a regular bus that you might be used to. The CAN bus is message-based, not module-based. This means that every message has an address and every module on the bus can receive it at the same time. Messages are not directed from module to module; they are directed at everyone. When a module receives a message it decides whether it is needed or not.

If you listen to the CAN bus you will hear every message that is being transmitted all the time. It is up to your code to filter out what you need. Most integral CAN modules have a way to read in wanted messages and discard unwanted ones.

The CAN bus is wired directly to the OBD port. If memory serves me correctly, pin 7 and 14. The gateway you mention is not between the OBD port and the CAN bus; it is between the CAN bus and the other slower buses in the car.