The Virtex-5 GTP RocketIO receiver can track Spread Spectrum Clocking (SSC), though certain conditions need to exist. This answer record discusses the limitations that exist in interfacing the GTP with spread-spectrum signaling.
When interfacing the GTP receiver to a spread-spectrum signal, the attribute PLL_RXDIVSEL_OUT must be set to "1." This limits the line rates that can be received while using spread spectrum clocking to those greater than 2 Gb/s. Using a setting of "2" or "4" can potentially lead to the Clock and Data Recovery (CDR) circuitry not tracking the incoming data accurately, leading to errors in the incoming data.
For information regarding the use of spread-spectrum clocking with PCI Express, please refer to (Xilinx Answer 19760).