The "A112.18.1M Canada" that you refer to is simply the ASME standard that the valves are constructed to.
As to the leak...it depends. Sometimes people mix gas and water ball valves. Some valves are repairable, some are not.
Many of the valves have two sweated fittings, one on each end. One female end is cast into the body of the valve's housing. A portion of the other end is cast into a hex shape and threads into the body.
It's doubtful that the threads on the two-part body came loose. The most common damage is damage to the rubber ring inside the casting that the ball valve rides in. If the valve wasn't opened when it was sweated in place, or if the valve was overheated when it was sweated, the rubber could have been deformed. Smetimes this damage doesn't become apparent until the valve has been cycled a few times.
Not much to do, really...some valves have through nuts on the handle. Try snugging up that nut, though for the majority of these valves that nut only holds the handle on to the stem. If you do have the two-part threaded body, try tightening that as well...though that's a tough joint to move.
How old is the valve? Has it been cycled, or did it just start leaking on its own?
Tearing down the manifold for R&R may be a pain, but it may also be your best bet at a fix.