I got it on steam with dlc classes such as thaumaturge and the Arcane Mechanic Does GOG version has them in some sort?

If you are playing the Final Cut version on GOG, then you get six different classes to choose from when you create a character, and no more than that, and they are:

1. Protector
2. Bounty Hunter
3. Elementalist
4. Umbralist
5. Phlogistoneer
6. Constructor

The classes you named in the original post are not available to choose in the Final Cut version on GOG. I assume that they are also not in the Final Cut version on Steam either, though.

However, if you buy the GOG version of Final Cut, then it also includes the original 3 games as they were before they were re-worked by the devs into the Final Cut version.

So it's possible the GOG version does still have those classes you named, if you were not to install the Final Cut version, and instead install and play each of the 3 games separately via their own separate installers that GOG also provides to you if you buy the Final Cut version on GOG.

However, I don't want to install those games separately in order to check for sure as to whether they do or not have those classes that you named, but maybe someone else who does play the separate, non-Final Cut versions will come to this thread and answer your question eventually.