This has already been discussed by SmartThings staff. Nortek (who make the 2gig devices) often use the same case for multiple different devices, but the insides may be very different including different firmware, different radios, etc.
In this case, you’re right that it is using the same case and starting from the same UI for the security portion, but the new SmartThings/ADT device is not just a rebranded GC3. Both the hardware and the software inside are very different.
If you want to try and use the standard GC3 with SmartThings you have all the same problems you’ve always had. Although technically SmartThings can be added as a zwave secondary controller to a network where another device is the primary, it lacks some of the typical Z wave controller management command sets. For example, if after you add it you then add a new device to the primary, you have to remove the SmartThings hub from the network and then re-add it from scratch or it won’t know about the new device.
It also may not get status updates on individual devices that are added to the primary.
It won’t be able to share any information about virtual devices or zigbee devices with the primary system.
Other Z wave utilities may not run properly across the consolidated network.
So while there are some community members who are using a SmartThings hub as a secondary to another primary Z wave controller, officially SmartThings discourages this practice and SmartThings support will not give you any help if you run into problems. 
SmartThings strongly discourages adding the Hub to another Z-Wave network. We cannot offer support for disconnected Z-Wave devices or the inability to add devices through the Hub as a result of including the Hub into another Z-Wave network
So…
The new ADT/SmartThings security System tablet controller is not the same device as the 2gig GC3 although it uses some of the same component parts including the physical case.
And if you want to try to directly integrate the 2gig branded device with SmartThings, you can try, but the results will probably not be what you expect, SmartThings support will not give you any help, and you may have to rebuild your network a couple of times from scratch.