It means that at the moment of trigger, when you are forcing just enough current into the gate to guarantee triggering of the thyristor , the gate voltage will be no higher than 1.3V (at 25°C).
It may be higher at lower temperature, and it may be higher if you force more current into the gate.
So, suppose you have a +5V supply that yields minimum 4.5V after a transistor switch drop and tolerance, you can calculate the required gate current-limiting resistor from Rgt = Igt/(4.5-1.3V)
simulate this circuit – Schematic created using CircuitLab
