Ghostring does not work on regular monster attacks in PvM.
However its use in PvM is to counter big scary moves like Earthquake, you can pretty much tank any EQ in the game with GR and an alice shield.
In WoE its to lessen damage from Acid Demo, Matyr's Reckoning (sacrifice), Asura, and uhhhh other dummies using neutral attacks when elemental options are available to you.
GR is worth a lot since it is very useful, and you can't pop them open from OCA's anymore. :D