Alter the top-level XDC LOC to the GT locations. To get the paths for the GT locations, first elaborate the design then enter this Tcl command in the Vivado Tcl console:
get_cells -hierarchical -filter {NAME=~*gtxe2_i*}
This command is for a device containing GTXE2s and can be modified for gtpe2_i or gthe2_i as appropriate.