"Rishi Durvasa, I don't understand what do you mean why this, Can you please explain it to me?"
Gandhari came out of her shock, As she did she quickly asked Rishi Durvasa what did he mean by her son being immortal.
She, contrary to the imagination, was quite scared and worried other than being happy about knowing this information. This looks odd in the eyes of others but she who grew up hearing the stories and personally experiencing them, Knows that the boon is just another form of curse.
She just got one boon, That too not that strong just to be blessed with being the mother of 100 children, The reason also was not bad ones All she wanted her daughter to experience the love and care she given by her own 100 brothers.
There was no ill intent, There was not treachery behind asking that boon, Just a pure emotion of a mother wanting good for her daughter but what happened? That boon became the reason why her whole family got destroyed.