>>Is your suggestion, I just install the Win8-drivers provided by AMD?
Exactly. I had a model in my office with the same config - I had to use an external GPU to scale up to 1920. It wasn't offering 1920 even with Windows 8 drivers(only up to 1366)
The highest Gigabyte provides is Windows 7 though.
https://download.gigabyte.com/FileList/Driver/m...
Disclaimer:
The link included is a non-Microsoft website that would provide accurate and safe information. Watch out for ads on the site which are frequently classified as a PUP (Potentially Unwanted Products). There is no need to buy paid products to fix your computers as they do more harm than good sometimes.