The result is yes, my 4x GPU (3GB Gigabyte GTX 1060 Gaming G1) setup on Gigabyte GA-Z170-Gaming K3 has worked!

1) I have upgraded the BIOS to latest version currently available - F22.

2) I installed Win10Pro x64 in UEFI mode (it was incorrectly installed initially - a hint from my observations: when you remove all logical partitions from the drive, and when installing in UEFI mode, and when the system allocates space for the installation - it creates at least 3 additional logical volumes / partitions. If booted up/installed in any other mode, the system creates only two such partitions! If you see that only two partitions on the hard disk were created before the Windows installation, then you booted up incorrectly, not in UEFI mode). For booting up from USB drive into UEFI mode, I went to the BIOS, in the section where you can save its settings - the BIOS shows the devices connected to the motherboard in the Boot Override section, and among all the options chose where the word UEFI is present.

3) before booting into Windows, I went to BIOS and disconnected EVERYTHING that I do not use, like SATA ports where nothing is connected and all other ports like serial, audio panel, onboard graphics, etc. In M.I.T. section of BIOS I tuned PCI to Gen 2, but works in Gen 1 mode too.

4) after all these settings and installation of Windows - I connected ONLY one GPU (3GB Gigabyte GTX 1060 Gaming G1) with the riser to the slot PCIe x1 which is closest to the CPU (in BIOS I've set this port to be recognized as the first), immediately updated the chipset driver (downloaded from the manufacturer's website), and the latest nVidia driver. After that, I connected 3 other GPUs.

5) But then only 3 out of 4 GPUs were recognized. Therefore, I can not further reliably say that this was the solution, but I connected another, fifth GPU (Gigabyte GTX 1060 Windforce 2 3GB) through a new riser to an open slot PCIe x1, loaded the system, and it detected this fifth card. In the system was defined as the fourth in the list of adapters. Using the GPU-Z utility, I checked each card using the Lookup function to see which card was allocated at number 4 - when clicking Lookup in GPU-Z, the browser opens with the photo of the card that you are detecting/looking up, so after checking each one in turn, I immediately realized that it was the fifth new GPU that was detected. Next step, I decided this way - I will disconnect only the GPU from the old riser that was not detected (I left the PCI x1 adapter in that slot), and I will swap it with the new fifth card, which was detected on a new riser with a PCIe x1 adapter. Another detail - for this new riser I used a different power source - the SATA to MOLEX adapter (the previous riser, where GPU was not detected, was powered directly from the EVGA Supernova 650W G2 80+ Gold Modular) via a modular cable with 3 MOLEX connectors).The fifth GPU - I have put it back in the box.

6) rebooted the system - and YES I have had all 4x Gigabyte GTX 1060 Gaming G1 3GB working.

7) The old riser with a PCIe x1 adapter (where GPU was not previously recognized) I have pulled it out during another reboot.