upd. solution found: https://www.techpowerup.com/vgabios/208828/208828 thanks darkneorus!

use nvflash to flash your card which is not asus or gigabyte, you'll get all 8GB working on board.

anyone knows how to open all 8GB VRAM on P104-100 to mine Grin31?

it should be done with bios modding.

https://i.imgur.com/FOiWLxd.jpg
https://i.imgur.com/iWl7ibt.jpg
https://i.imgur.com/M7Gu0MJ.jpg