You can use a gigabit transceiver output pin (e.g., a recovered clock) as the primary clock root as shown in the following figure.
Figure 1. create_clock on a Primitive Pin
create_clock -name txclk -period 6.667 [get_pins gt0/TXOUTCLK]
Recommended: For designs that target 7 series devices, Xilinx recommends also defining the GT incoming clocks, because the Vivado tools calculate the expected clocks on the GT output pins and compare these clocks with the user-created clocks. If the clocks differ or if the incoming clocks to the GT are missing, the tools issue a methodology check warning.
Note: For designs that target UltraScale™ and UltraScale+™ devices, Xilinx does not recommend defining a primary clock on the output of GTs, because GT clocks are automatically derived when the REFCLK input clocks are defined.