Gpu connected: black screen to "no signal"

[escharf18]

Hey all, I just completed my first pc build. However I would say I know a bit more about this stuff than the average building noob. It is as follows:

-MSI Z170a M5 motherboard
-Intel Core i5 6600k
-Nvidia GTX 1070 FE
-16gb Gskill Ripjaws ddr4
-Crucial MX500 250gb M.2 SATA SSD
-Corsair H55 water cooler
-Corsair CX550M 550 watt PSU

So I'll tell you what works. When I plug a working HMDI cable into the motherboard, the PC boots up, lights go on, GPU fans spin and illuminates the text on the side. Keyboard and mouse light up. I am able to get into Windows 10 but the GPU is not in device manager or anywhere else for that matter. Yes I have checked hidden devices and all that, still nothing. Probably because I'm plugging into the motherboard and not GPU, right?
Well, when I disable Intel graphics and plug hdmi or displayport into the GPU, the monitor cycles through its inputs and settles on no signal. I have already gone into BIOS and made sure it is set to default to PEG (not the other one). Note, I do not have any drivers for the GPU because Nvidia refuses to download drivers seeing as it cannot find the GPU in the first place. Also, when I unplug the PCI-e 8 pin cable from the GPU but leave it in the motherboard, plugging the HDMI into the motherboard gives me display.
Oh, and sometimes when I go into device manager (plugged into motherboard so I can actually see stuff on screen) the gpu RARELY comes up under hidden devices but Windows throws a Code 45 (telling me to "connect the hardware"). I haven't been able to get even that to happen recently though. I have also tried all this while plugging the gpu into the other PCI-e x16 slot and nothing improves.
I'm basically out of ideas. The only thing I haven't done is reset the motherboard or BIOS. Is it a bad motherboard? Bad GPU? I don't really have another build to just these throw various parts into to check if they work somewhere else. Any ideas? Thanks

TLDR: my damn GPU won't show up anywhere and I've tried just about everything. HELP

Edit: in BIOS board explorer, all PCI-e slots show as empty when the GPU is plugged into one of them. This is probably important. Also, when I move the GPU to the other PCI-e slot, I get no signal when plugging HDMI into either the motherboard or GPU.

 

[EliteAP]

No, you need to leave the INTEL HD GRAPHICS enabled.

If you don't, you will get a BLACK NO SIGNAL SCREEN.

Plug the HDMI into the GPU and leave the HDMI in there, make sure your GPU connections (power) are in securely and ensure the graphics card is SEATED properly in the PCI-Express lane.

Then check if it's showing in Device Manager.

 

[escharf18]

When I plug the HDMI into the gpu nothing shows up on the screen which is the problem I'm having. I've enabled Intel graphics and nothing shows up on screen when I plug either hdmi or displayport into the gpu.

Basically my computer is acting like the gpu does not exist, is not plugged in, and is not able to display anything through it. Even though the gpu is receiving power and is plugged into the first pcie slot.