I think the key interval here is 20 minutes, rather than 2. This is how often the sensor will send its updates, via Bluetooth, under normal circumstances. The data included in that update is sampled every 2 minutes. In other words a standard update will include 10 samples. The update via bluetooth will take place immediately, if a threshold is exceeded.
Given that the sensor can be 'quiet' for quite a long period, the concept of which gateway a sensor is 'connected to' doesn't really exist. The most relevant gateway is probably the one it most recently connected to, but it may not use that gateway, the next time it connects.