The speed (either 2.5 GT/s or 5 GT/s or 8 GT/s) is listed in the PCI section, but only if your DSDT has the changes that indicate the PCIe slot devices. And anyway, the graphics card may idle at 2.5 GT/s and then speed up when doing rendering, so in that case you need to read the actual PCIe status register while doing rendering to get the speed. In mac OS, you would use lspci to dump the registers while the card is doing 3d work, find the register value, and interpret the bit fields using the PCI specification. In Windows, you would use GPU-Z, click the "?" icon to start a rendering test, which should cause the speed to inrease from the idle value to the full speed value.