Your motherboard doesn't not seem to support any modes but 16+4: https://www.asus.com/Motherboards/Z170-P/specifications/
So if you install a graphic card on the gray x16 slot, you can still install some other card (graphic or not) on the black x16 slot and get x4 bandwidth (or lower depends on the card itself).
Each pci-e 3.0 lane is 8GT/s, i.e. 985MB/s: https://en.wikipedia.org/wiki/PCI_Express
I have NOT seen from the aforementioned spec or the user manual that the pci-e x1 slots / pci slots / m.2 slot shares with the pci-e x16 slots the 20 lanes.