Jens:
I you could do two things. first I would point the mount as closely to the polestar as you can with you phone compass or other tools and dial in the declination from the area you're in. you can research that. Then go through the 3 Star alignment in the synscan app. Once you've done that you can start polar alignment on a star, the synscan app will tell you what to do. that worked pretty well.
You can also go via the ZWO asi air to do an all sky polar alignment, that works fairly well too.
The important thing is that you do the setup of the scope step by step and don't skip one. then it should work pretty well.
the Coordinates are very important as well.
If that doesn't work at all I'd highly suggest you getting the asi air or hook it up to a laptop to plateslove, so if it is off, it will recognize that and correct itself until
it is spot on.
I struggled as well in the beginning but with a bit of patience I got it to work with the GTI.
I second this, as well as many other people have said. The asi Air is just a very easy and convenient platform for everything, and I find my polar alignments to be spot on every time.
I do however have one tip for you regarding the accuracy of the GTI's go to: I find that if you do your 3 star alignment near the area where your object of interest is, (ex. West) then it will be more accurate than if you align some other direction away from your target (ex. North or south). Sometimes I found that a one star alignment with a star right next to the target was better than a 3 star alignment of 3 bright stars spread out over the entire sky.
Hope this helps! CS.