Nothing in GSX has changed in the last update in relationship to the Concord but, of course, without knowing when you flew it the last time and it supposedly worked, it's not possible to understand why it might not work today.
The only thing special I see on the Concorde default configuration, that hasn't changed in years, is that pushback won't start unless you start engines 2 and 3 and GSX will tell you to do so.
n the past when pushback was requested I was first prompted by GSX to start engines 2 and 3 and then it connected and requested that I release brakes, etc. Now pushback begins before I can start the engines
GSX pushback will never start until *after* you release the Parking brakes so, in this case, you should start engines 2 and 3 and don't release the parking brakes until they are running.