I recently purchased a STM32F7 Nucleo boards to play around with Chibios. These boards feature a integrated STlink debugger, I however do most of my development using a custom built debugger based on the blackmagic probe. This has the advantage of being easily connectable to GDB via emulating gdbserver on it’s virtual serial port.
Unfortunately there doesn’t look like there is a documented way of connecting a 3rd party debugger to the Nucleo board but reading the schematics showed an answer in the form of the CN4 jumper block.
In order to connect a the black magic probe to the Nucleo board simply remove the jumpers from CN4 on the ST-Link . You can then access the SWDIO pins on the main MCU as shown in the list below.
You will also need to connect the ground pin of the blackmagic probe to a ground pin on the Nucleo board (I used one of the ground pins which have jumpers on them which are used for feet on the board.
And here is a picture of the board.