Triggering of TRIAC for Crowbar Protection
The gate requires at most 35mA to trigger at a junction temperature of 25°C. It could be more like 53mA at -40°C. That applies to Q1, Q2 and Q3. 

It is guaranteed not to trigger with 1.75mA or less gate current. Again at 25°C. That could be around half that at high temperature. Q1, Q2 and Q3. 
The current is the independent variable. They are saying that when you drive the gate with enough current to trigger the triac (exact conditions stated), at that current, the gate voltage will not exceed 1.3V (at 25°C junction temperature). It could be more like 1.65V max at -40°C. 


You should drive the gate with a controlled current, knowing that the gate voltage at trigger can be variable. Usually that means a series resistor. In the case of the crowbar, the triac removes drive when it crowbars the supply, so provided that happens fast enough that the peak current does not exceed the maximum the triac won't be damaged. 



In essence the way you use those numbers is as follows. Suppose you are driving the gate with a switch (such as a transistor) and series resistor R from a fixed supply voltage Vs. The switch has a voltage drop of Vsw

The gate current is Ig = (Vs - (Vsw + Vgt))/R

To ensure the triac will trigger under all conditions (Ig >= Igt) you need to look at the worst-case values for Igt, Vs, Vsw, Vgt and R. 

That will likely occur at the minimum junction temperature for worst-case Igt, minimum Vs, maximum Vsw (at Igt) and maximum R.