How to choose a "gate current limiting resistor" for an SCR in a crowbar circuit using gate voltage and gate current from SCR's Datasheet?
Isn't the current enough? Is it important to have this 0.6V or 1.5V
  supplied to the gate?


The SCR gate needs 1.5 volts to guarantee a 32 mA drive - the gate has a non-linear input impedance and, as far as I can tell you'll need to account for the 1.5V needed by the device's gate when the gate is drawing 32 mA.

If 32 mA is passing thru the 10 ohm resistor then this means that the input voltage to the left side of 10 ohm resistor is 1.5 volts + 32 mA * 10 ohm = 1.82 volts. If the zener is 7 volt rated then the circuit will crowbar at 8.82 volts.

As an aside - why are you putting a crowbar circuit on a 5V linear regulator like the 7805?