You could release your application commercially right now if you like to, but you have to make the source code available as well. The buyers/users of your software must also be informed that they may receive the source code on request. And your application should contain a file named "copying" or "copyright" containing information about the license and the authors of the different parts of the application, including mbarcode and libraries like zbar, libdmtx, etc.

The easiest way to make the code available is to host it somewhere like Garage or Github.

The GPL license does not require you to give away the binaries (the compiled application) for free, even if the source must be available free at charge. This means that you are in a perfectly fine position to make money from your application even if it is open source The GPL license does not have any restrictions on commercial usage other than that the source code must be available. So feel free to commercialize your application

I don't remember exactly everything that the GPL license requires, so if you are unsure about anything, check the GPL FAQ or have a look at the license text.

Licensing the code from mbarcode "commercially" as in removing the GPL requirements is more likely a big project. First of all we would have to gather consent from all contributors to mbarcode (at least those whose code is still in use), and we would have to get consent from the developers behind libdmtx, which is also licensed as GPL. A lot of people have put an effort into this and I don't think they (including me) will license it commercially without getting some kind of compensation.

The reason why GPL is chosen by most people (again, including me) is its strength to make sure that when someone builds upon what you've created, the changes which are made are shared so that others can build upon that again.