How is Gate Trigger Current determined in a "dimming" circuit?
The Diac is a highly non-linear device. Below the diac breakover voltage, minimal current flows. When the breakover is reached, the diac conducts strongly and discharges C2 through the Triac gate.
The Triac gate current should easily exceed the minimum trigger current. How high the current will be is not as easy to calculate. Some diac specifications will give a maximum current under certain conditions. If you tell us your diac part number, we may be able calculate the maximum current.
A power triac can handle a fairly large gate current momentarily. A series resistor to limit the current is usually not needed.