BLE works over a system of services and characteristics. These are basically identifiers, allowing people to operate different ones for different purposes. So one service can have an ID and then have multiple characteristics, also with ID's, that can be used to send/receive data between the central and peripheral. They are very loosely analagous to sockets in the IP world, so you can think of one service (UDP, TCP, etc.) and then multiple characteristics (Socket or Port number) inside of the service. This is just to show how it works, BLE and IP are quite different.
So, their device could potentially send raw data over the BLE characteristics, I have seen a lot of simple devices do this. If that is what they are doing, then it shouldn't be too difficult to figure it out. But yes, if they built an application on top of the BLE layer, you would have to contact them.
For example, bluz uses a Service and Characteristics to send data to/from the cloud. However, we encrypt the data on the device before it is sent over the air, so anyone trying to read it would only get gibberish. I don't know what their devices do, so you would certainly need to ask them or just try it. If they only send raw values unencrypted over the air with no special application logic, it would be pretty straight forward.
To reprogram the gateway to work this way, you would need to alter the firmware at the Nordic SDK level. This is certainly doable, but will require C/C++ experience as well as some additional hardware devices (JTAG programmer for example). Though the programmer isn't 100% necessary, I would highly recommend it if you wanted to flash new system firmware regularly, which you would. Also, a UART to USB converter would help you debug. All can be purchased for well under $50 total, so not a big expense. The Nordic SDK is very well documented and there are lots of examples and a great community there as well, so if you have some C/C++ experience and want to give it a shot, it shouldn't be too bad, as long as their device will play nicely