Ok, I got around to adding the RSSI measured from the DK: https://github.com/bluzDK/bluzDK-firmware/commit/3c863b1bfd407746a3c0a40abaea6ca6bf0dcea3
This will be in the next release. I am working on adding it to the gateway side as well, it is a little different and I am trying to roll this issue in with issue #42 as well. That may take a bit longer to get working.
Note that is the TX power of the DK is set to the same value as the gateway (which is 0dBm), then the RSSI measurements should basically be reciprocal. So if the DK is measuring -75dBm, then the gateway should see the same if the DK is transmitting at 0dBm.
Likewise, you can calculate the RSSI at the gateway. If the RSSI at the DK is -75dBm and the DK is transmitting at -8dBm, then the gateway should be seeing -83dBm. This won't be 100% accurate, but should be a good approximation.