Gart is for AGP ...
PCIe Graphics & AGP
In addition to the bandwidth considerations mentioned above, there are several other differences between AGP and PCIe.
By definition, AGP requires a chipset with a graphics address relocation table (GART), which provides a linear view of nonlinear system memory to the graphics device. PCIe, however, requires that the memory linearization hardware exist on the graphics device itself instead of on the chipset. Consequently, driver support for memory linearization in PCIe must exist in the video driver, instead of as an AGP-style separate GART miniport driver. Graphics hardware vendors who want to use nonlocal video memory in their Windows XP driver model (XPDM) drivers must implement both memory linearization hardware and the corresponding software. All PCIe graphics adapters that are compatible with the WDDM must support memory linearization in hardware and software.
AGP was dedicated to graphics adapters, and no other device class used it. PCIe is intended to be used by all device classes that previously used PCI. With AGP, a number of video drivers were directly programming the chipset, which gave rise to severe ill effects such as crashing and memory corruption in the graphics stack. Because PCIe will be used for all devices in the system, it is even more important that video drivers not program the chipset directly."