Your RX580 seems to be running at x8, when it is installed in the only x16 slot on your motherboard. Have you checked to ensure your graphics are setup correctly in your bios. With the PCIe slot/card selected as the default graphics option. Any Integrated graphics options should be disabled or set to the discrete card.
I think your motherboard requires an AMD APU to be present for the system to run the discrete graphics card at PCIe 3.0 and x16, at least that is what I understand when reading the information on the Gigabyte specification page for your motherboard.
You are of course connecting the display to one of the RX580's display-out connectors, yes?
You aren't connecting the display to the VGA or DVI connector on the rear I/O plate (motherboard)?
FaceTime and Messages etc, require a working Apple iCloud account.
Apple's iCloud requires a working SMBIOS data set, which you can generate as detailed above. Don't be lazy, generate your own SMBIOS data set.
I don't understand why your graphics card is not showing correctly in About this Mac and the System Information > Graphics/Displays report. Your motherboard only has the one x16 PCIe slot, so it is not likely to be an issue with the PCIe slot you are using, your bios shows the AMD Integrated GFX is disabled and Initial graphics as the PCIe slot, you have Lilu.kext and WhateverGreen.kext in your OC/Kexts folder and both kexts are enabled in your config.plist.
While the GPU and display details are not shown in About this Mac and the system information report, this does not prevent your AMD GPU from working. I assume you can set your display to show the best resolution it supports, whether that is 1920x1080 or larger.
I can only assume the issue is related to the display connector and framebuffer the AMD GPU is using. To see if this is the case you would need to provide a copy of your IOReg.
How to Make a Copy of IOReg (Note 1)
1. IORegistryExplorer/File/Save a Copy As…. (no extension required)
2. Open file to verify (Note 2)
3. Attach to post (Note 3)
1, Open Downloads/IORegistryExplorer-SLRID_v10.6.3
2. Drag IORegistryExplorer to Applications
1. An IOReg.txt is not useful. It is not a "Copy of IOReg." Do not attach .txt files.
2. IOReg must open the file. If TextEdit opens, See Note 1.
3. Compress it (Finder/Compress ... )
Use the version of IORegistryExplorer below. Newer versions tend to corrupt the IOReg and make them unreadable.
The IOReg shows your RXC 580 as connected correctly. It also shows that you are connecting your display via a HDMI connector, from the fourth connector.
Does your RX580 have 2 x DisplayPort, 2 x HDMI and 1 x DVI connectors? As that is what is shown in your IOReg.
It shows your display connected to ATY,AMD,[email protected].
The connectors are named ATY, AMD,[email protected] to @4 (five connectors in total).
If you are connecting the display through a HDMI connector, then I would say the GPU not showing in About this Mac and the system report are purely cosmetic issues, and not to worry about them not showing correctly.
If you are not connecting the display through a HDMI connector, say if a DisplayPort is being used, then this shows that WhateverGreen.kext and the AMD framebuffer being used with your RX580 is not working correctly for your card. A custom Framebuffer may be required, but only if you are having issues with the display as it currently works.