The area in the figure below between VOL(max) and the dashed line is the Low Level Threshold Voltage. The area between VTT(min) and its corresponding dashed line is the High Level Threshold Voltage. These voltage ranges act as a buffer protecting the input from being corrupted. The Input Threshold Voltage prevents the input level from being misread as its complimentary signal or as undetermined.
Seldom is there a need for adjusting GTL Reference voltages to correct the output of the circuit. Proper resistor termination values along with maintaining minimum stub lengths of the onboard circuitry is usually enough to keep overshoot and ringback in check at the speeds we push for. Other issues that would affect the integrity of the output signal are countered by the increased VTT or VCORE voltage that is already being used by the time a need for GTL Reference voltage adjustments become a necessity.