Yes it is possible and works.
1) check that the pins match on the adapter and the media.
2) Update the BIOS before revision F6a https://www.gigabyte.com/MicroSite/462/images/BIOS-Z270P-D3.zip
3) Set some setting in the BIOS
http://www.gigabyte.ru/microsites/data/148/BIOS-setup-F22_step03.jpg
http://www.gigabyte.ru/microsites/data/148/BIOS-setup-F22_step04.jpg
http://www.gigabyte.ru/microsites/data/148/BIOS-setup-F22_step05.jpg
http://www.gigabyte.ru/microsites/data/148/BIOS-setup-F22_step06.jpg

4) Enjoy of working 7 GPUs