I've accidentally supplied the gate of an high power phase control thyristor (Vdrm=5200V and It(rms)=6500A) with a negative current (about two times the trigger current and four times the trigger voltage; ~900mA 12V DC) for a couple of seconds, a few times(4-5). The device apparently works fine when turned on by a positive gate pulse. Is it still safe to operate or it sustained internal damage being prone to failure?
You are in a very rarefied environment with such a massive power semiconductor.
It's certainly possible that something has been degraded as a result of exceeding the abs max ratings and it won't show up until you subject the SCR to something approaching full load, perhaps in hot spots leading to destruction or something like that.
I would guess (and maybe you don't know) that if the trigger current or turn-on time has changed as a result of this "event" the chances are much higher, but that's a guess and worth approximately what you paid for it. It may be safer to avoid using it where failure would be inconvenient or dangerous.
A thyristor is a semiconductor device, so relies on fairly precise dimensions and material doping qualities. Failure modes are numerous, from cracks to impurity precipitation. See this Panasonic reference for more details on semiconductor failure modes.
Thus, if the datasheet's absolute maximum values have been exceeded then damage is possible and may be difficult to detect. The issue with exceeding the negative gate maximum voltage is that diode junction breakdown occurs allowing excessive current to flow in the device. 900mA @ 12V is more than 10W of power dissipated in a junction operating in a unspecified state. If it was for a few microseconds I'd be less concerned, but for a couple of seconds I would consider the part suspect.
I would expect the part has suffered some damage that may lead to early failure or behaviour of out specification.
Sorry to be the bearer of bad news.