Hello @Petr Vanek,
thank you very much for your fast reply!!! You helped me a lot.
So for all other users here my workaround:
1. I model the Fixtures in the last Blenders Version 2.93.1 (because I like the new user-interface a lot more then the user-interface in blender v.2.79).
1.1. I model the fixtures topdown (the beam/light is pointing downwards).
2. I export the different parts of the 3D model (Base, Yoke, Head, Lens) separate as obj-files.
2.1. I installed Blender v.2.79 on a USB-Stick in this version I import the different obj-files (check the scale) and export then as 3ds files.
2.2. I import this 3ds file in the gdtf Fixture-Builders and everything is fine. In the gdtf Fixture-Builder and in grandMA3.