Hi Jjps
One of the main features of the DW1000 is generating a precise timestamp on SFD. This is done taking into account even small antenna and transmission line delays that can be calibrated.
Using a pin and recording the timestamp on an external MCU does not make a lot of sense, the time it takes for the signal to travel from the DW1000 to your MCU and for the MCU to detect the GPIO change will already greatly impact the accuracy.
It is not characterised and it doesn’t make a lot of sense to do so. I would guess the delay is insignificant relative to the IRQ / counter delay of your MCU. This delay is typically in the order of μs, the DW1000 deals in orders of ns.
To take full advantage of the DW1000, I suggest using the SPI interface to read the internal timestamp of the DW1000. This will give you a far more accurate synchronisation than what you can achieve with a GPIO pin.
If using SPI is not an option, you might want to try to characterise this yourself, the internal register of the DW1000 should be able to hep you with this.