This is a great question!
The nrf51822 on the gateway shield is connected to the Photon/Electron through 7 GPIO pins. 4 of these are used for SPI communication, the others are used for controlling data flow. We have a specific pin that we use to alert the Photon/Electron (which is the SPI Master) that data is available on the nrf51822 (which is the SPI Slave).
Theoretically, yes, you can use this pin, which will only go high when the nrf51822 wants to send data to the cloud, to wakeup the Photon/Electron, which could be sleeping otherwise. However, this can lead to interesting issues. For example, if the Photon/Electron takes a long time to connect back to the cloud, then data in the buffers on the nrf51822 could get overriden while it is waiting to send it.
To be honest, I hadn’t been planning on this for the most part. I’d always imagined the gateway as an always-on, constant-powered device. It would be plugged into the wall in a house or in some other reliable power source at all times. So power consumption wouldn’t be a critical issue with it.
Do people see making a battery powered gateway? Is this a concern of people? I would love to hear feedback on this and see peoples plans.