Inrush current switch with thyristor?
However I'm a bit unsure what will be the maximum allowable gate
  voltage for the thyristor.


I can see you are struggling to understand the data sheet so, maximum gate power in the table is specifed three ways: -


\$P_{GAV}\$ is an average and is a maximum value of 0.5 watts
\$P_{GM}\$ for 30 us has a maximum value of 10 watts
\$P_{GM}\$ for 300 us has a maximum value of 5 watts


Because you are using the gate once every several seconds I believe you should use the 5 watt figure to be a little conservative.

Looking at figure 5 and position 5 (5 watts) it suggests using a gate current of 6 amps and this results in a gate voltage of about 0.9 volts (hence about 5 watts).

Although the tables show a gate voltage of about 1.5 volts these, I believe are absolute maximums. The gate trigger currents shown just below those figures I believe should be shown as minimum values thus indicating they must be exceeded for guaranteed operation of the device at a certain speed. Remember also that the speed of the device is going to be faster at higher gate currents.

Also take note of the transient thermal impedance curves to see if you need any extra heatsinking. These demonstrate that you can rely on the thermal mass of the device for soaking up a single pulse of 100 ms duration and the temperature will rise at 0.24 degC per watt dissipated worst case. If the inrush current is over quicker then that makes heatsinking less problematic but, do take care of the steady state power dissipation because the thermal resistance is 0.6 degC per watt and if the device is operated for more than a second then heatsinking is likely to be needed but it all depends on the steady state current that flows.