I read in a recent news article that after the war, Gandhari cursed that Shakuni's kingdom ,i.e., Gandhar, will never have peace. Is this mentioned in the Mahabharat or it's just a folklore? Also this gives rise to another question, can someone be cursed after they have already died?
Her curse was to Krishna, not Shakuni.
First the reference from the scripture:
"Gandhari said ... Since deliberately, O slayer of Madhu, thou wert indifferent to this universal carnage, therefore, O mighty-armed one, thou shouldst reap the fruit of this act. By the little merit I have acquired through waiting dutifully on my husband, by that merit so difficult to attain, I shall curse thee, O wielder of the discus and the mace! Since thou wert indifferent to the Kurus and the Pandavas whilst they slew each other, therefore, O Govinda, thou shalt be the slayer of thy own kinsmen! In the thirty-sixth year from this, O slayer of Madhu, thou shalt, after causing the slaughter of thy kinsmen and friends and sons, perish by disgusting means in the wilderness. The ladies of thy race, deprived of sons, kinsmen, and friends, shall weep and cry even as these ladies of the Bharata race!’"