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brcm-pcie fd500000.pcie: link down

The issue is the missing PCIe Port, hence you have to disable the feature to get rid of this message. Create the file disable-pcie-overlay.dts with the following command:

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wget https://gist.githubusercontent.com/SirUli/de94ddb751b91adfa6809f286a18a382/raw/af010907360153f74edc8f65d415d717378bf037/gistfile1.txt -O /tmp/disable-pcie-overlay.dts

You must then convert the file using the following command:

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sudo dtc -@ -I dts -O dtb -o /boot/overlays/disable-pcie-overlay.dtbo /tmp/disable-pcie-overlay.dts

Then delete the file with the following command:

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rm /tmp/disable-pcie-overlay.dts

Finally open the boot config file using the following command:

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sudo nano /boot/config.txt

And add the following line at the top:

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dtoverlay=disable-pcie-overlay

Then save using CTRL+X which will ask you to save. Enter Y and ENTER to save. On the next reboot that message from above will be gone.

GPIO stands for General-Purpose Input/Output. It refers to the pins on a microcontroller or single-board computer, like the Raspberry Pi, that can be programmed to serve as either inputs or outputs. These pins enable the device to interact with the external world by receiving or transmitting digital signals. Through software programming, developers can configure GPIO pins to read data from sensors (inputs) or send signals to actuators (outputs), facilitating versatile applications like the control of fans, lights or other 3D printing accessories.

Thankfully the pinout is the same on the Manta M4P which is why i was able to extract the data from there  (page 9):

BIGTREETECH describes the pinout of the CM4 the same as the one of the Raspberry pi. However when being used with the CB1, the Pinout Changes. I extracted that information and then used various sources of research to find the corresponding GPIO port: This is the final result:

Update: Now i finally found a mapping in the CB1 repo - check there  if you want to see the mapping for the BTT PI or CB1 MMC version.