I don't know if the CFB Gagetown Shooting Association does much if any longer range shooting. I do know that they do pistol shooting, and hunter sight-in days.
The Royal NB Rifle Association Fullbore Section runs a full summertime shooting programme, and our principal range is the 900m Batouche Range at CFB Gagetown. It'll be next May before we're shooting again, but I'd be pleased to give you any info you might be interested in.
There is also a club in Springfield (about 45 mins from Oromocto) that has a 600m and 1000m rifle range; quite a few of our members are from CFB Gagetown.