I don't think there is a formal JW doctrine of grace. Certainly the JWs would be accused of "works-righeousness" by the heirs of the Reformation. The Greek word "charis," usually translated "grace" in English Bibles, is translated as "undeserved kindness" in the NWT. (I don't think Luther would be unhappy with that!) But with regard to the apostle Paul's discussions of justification (or being "declared righteous" - NWT), it is claimed that this specifically refers to the 144,000 with the heavenly hope. They have righteousness imputed to them to enable them to run for the prize of the heavenly calling, and technically this is a 'justification by faith' because they have done nothing to earn it, but they are expected to perform plenty of good works thereafter.
The "other sheep," with the earthly hope (the majority), may currently be justified as "friends of God" similar to Abraham, but their actual justification will be a gradual restoration to human perfection during the Millennium (and they will be declared righteous after passing the final test at the Millennium's end), all the while having obeyed whatever God's law will be during that time.
Justification on any plane is ultimately due to the ransom sacrifice of Jesus Christ, and in that sense there is a belief in the grace of God. If confronted with the Reformation doctrine of salavation by grace alone through faith, JWs will instinctively turn to James' statement that "faith without works is dead." If this is understood to mean that true faith is proven by works, then it is a true evangelical statement. Unfortunately, there are too many who think they are "working their way through Armageddon." There has always been the tendency in this religion to avoid cheapening grace, and to emphasize works.
Certainly, there is no belief in eternal security, or "once saved, always saved." No one believes that they have been predestinated to salvation (predestination being that of a class rather than individuals), or that God has put them in a position where they cannot possibly be lost. Nor is there a sacramental grace such as in pre-Reformation traditions.
I hope this answers your question.
Justin