Not all of the GTs for some devices are shown in the Vivado tool's Device View window.
An example is with the xc7vx415tffg1158-2 which contains 48 GTHs, only 20 of them are visible in the Vivado 2013.2 tool Device View window; the rest are greyed out.
Why is this occurring?
Based on specific device packages, GTs are not bonded, so it will be shown as greyed out. This is the same concept that is used for IOBs that are not bonded.
If a design is created using the Transceiver Wizard, selecting the GTs that are greyed out in the Device view, opening the Device View after implementing the design will highlight the selected GTs. However, the Transceiver Wizard is not package-aware and, although it will let you select these GTs, the design will error in DRC as you cannot use an un-bonded GT in a design.