What does this IGBT data sheet mean by "transient" gate voltage?
Nooo... I would definitely not allow the gate voltage to approach that limit with 150 milliseconds duration. 

Here is an ON Semiconductor application note where they include a data sheet spec of the conditions. 



5 microseconds and duty cycle (0.1%?). 

Note also the failure mechanism, which is partly thermal, so (as the lawyers say) time is of the essence. 


  The voltage across the dielectric, between the gate and
  emitter can cause tunneling of carriers through the dielectric
  if it exceeds the leakage limit, especially where
  imperfections (traps) exist. This tunneling creates heat,
  which if allowed to continue for a sufficient period of time,
  can cause damage to the oxide which in turn creates more
  traps. This process can quickly increase to the point where
  significant damage occurs in the gate oxide. It should be
  apparent that at higher gate voltages it is necessary to limit
  the duration of the event to assure that the temperature rise
  due to the tunneling electrons does not exceed a safe level.
  
  If the oxide becomes hot enough to cause damage, that damage will be
  cumulative. This is why both the time and duty ratio of the transient
  event are included on the data sheet. Over a period of time this
  damage can cause the threshold to shift lower which may cause improper
  operation of the circuit or in extreme cases can cause a failure of
  the gate oxide.
  
  ON Semiconductor IGBTs are tested in qualification testing, at levels
  well above the transient voltage rating to assure that this is a safe
  transient level for the gate-to-emitter voltage.
  
  The DC rating is a very conservative gate voltage level and no
  tunneling or other degradation will occur at or below that operational
  level.