GTLRef Voltage (Gunning Transceiver Logic: doesn't that sound cool!) is the reference voltage the CPU uses to determine high or low signals (on or off) for data on the front side bus (FSB). The standard settings are set to “optimum” by DFI, but try adding to it in small quantities if you hit a wall with everything else. Kentsfield Quad Core CPUs are supported and require the four GTLRef voltages which DFI have integrated in 1&2 and 3&4. Pay particular attention to these settings if you're overclocking a Quad Core CPU.
The typical GTL switches between 0.4V and 1.2V when sending data (off and on), so increasing the value means the signal to noise ratio is stricter: allowing less difference between the data signal and that of the increased noise from increased voltage along the FSB traces between northbridge and CPU. If the standard GTLRef is 0.8V and is increased to 0.9V it means there's a greater difference between the stock 0.4V and reference voltage. But typically when overclocking the 1.2V VTT signal voltage will be increased (Intel rates 1.55V as an upper limit to VTT for it's processors), increasing the upper limit too. There is a greater overall difference allowing for more noise and a stricter data edge on the clock when increasing the frequency: 0.4V (off) - 0.9V (Ref) - 1.3V (on). It’s all about obtaining a balance between the two to filter the actual signals propagating through.
The CPU Core GTLRef values range from 1 to 255, with each value representing approximately 0.00153V, so the stock value of 48 represents a cut off voltage of 0.70V, out of typically 1.21V. Increasing this offers values of typically: 72 = 0.74V, 96 = 0.77V, 120 = 0.81V.
The Northbridge GTLRef values also range from 1 to 255 but each value represents 0.00146V, so a stock value of 128 = 0.86V. Increasing this offers values of typically: 144 = 0.88V, 160 = 0.91V, 176 = 0.93V.