Hi,
GPIO3_PP.03 is 443 for TX2. For Jetson Nano, please check #3.
There is rule of calculating the gpio number, but don’t have tool. You may let us know if you would like to set other pins as gpio pins. We can suggest next.
If you would like to use gpio pins to control hardware components, you may look for pins near the hardware components. Some pins are with multi-function such as PL4, PL5. The two pins are for USB functions. If you need USB functions on your custom board, We would suggest pick other pins as gpio.
This is jetson nano forum board. For jetson nano, it is using TX1 based chip so the gpio header file should be tegra-gpio.h. The offset of them starts at 0.