The 1Gbps line rate does not work when targeting the Virtex-5 LX20T device.This is not a limitation of the GTP transceiver inside the device, but a limitation of the IBERT core. Since the IBERT core is intended to be used as a method of tuning the PMA inside the GT for the board, the best work-around is to use a 2Gbps line rate instead and tune the transceiver using the higher line rate.