Why does the documentation suggests to leave the unused GT I/O ports connected to GND, even though Xilinx development boards have all available GT I/Os floating?
The user guide (UG476) provides all of the required details about how to connect unused GT I/Os.
A floating I/O is needed on development boards because any GT I/O must be accessible for tests and measurements.
If a particular GT is not used, this does conflict with the user guide recommendation.
The user guide also suggests to ground unused GT I/Os because this is good design practice.
However, an I/O that is floating or (that is a similar condition) connected to a decoupling capacitor will not break just because it is floating.