It was confirmed in the preview for episode 103 that it will be the series finale. It will probably have D.Gray-man just end like the anime companies had Inuyasha, Zatch Bell, Bobobo, Ramna, Tsubasa, and many others.
However, hope is not lost because there can be a renewal later on like there have been for some other animes, such as Slayers, Fullmetal Alchemist (currently a new anime is in production), Tales of Symphonia: The Animation, and xxxHolic. It could also be brought back to life through OVAs like Tsubasa, Ramna, and HunterXHunter.
I am certain that with enough fan support (which D.Gray-man does have), I think we may see something in the future.