Gabrielle's Dagger was intended to be used to murder Steven Deschain. Kingson gave it to Gabrielle Deschain to use. However, she never uses it on her husband presumably having second thoughts about murdering him.
The dagger was made by Marten Broadcloak and a small scratch from it would cause the victim to bleed out of every pore causing extreme agony and death weeks later.
It is unknown what happens to the dagger after Gabrielle is killed.