CONFIG_GART_IOMMU is not available for thedefaultarchitecture x86.
Result is shown for architecture 64BIT

Old AMD GART IOMMU support

modulename: amd_gart_64.ko


Linux Kernel Configuration
└─>Processor type and features
└─>Old AMD GART IOMMU support
In linux kernel since version 2.6.12  
Provides a driver for older AMD Athlon64/Opteron/Turion/Sempron
GART based hardware IOMMUs.

The GART supports full DMA access for devices with 32-bit access
limitations, on systems with more than 3 GB. This is usually needed
for USB, sound, many IDE/SATA chipsets and some other devices.

Newer systems typically have a modern AMD IOMMU, supported via
the CONFIG_AMD_IOMMU=y config option.

In normal configurations this driver is only active when needed:
there's more than 3 GB of memory and the system contains a
32-bit limited device.

If unsure, say Y.