Agree, Des.
Not only is it easy to sing--it gets easier the more you do it. There's something about the way the Propers-chants are constructed which "works" in the memory banks so that even when reading unfamiliar Propers one almost 'knows where to go' in advance.
It depends on what "chant" is. If we're talking "Missa de Angelis" or similar, the scales used are familiar as they are derived mostly from "major". Even so the Latin would be a challenge for the uninitiated.
If we're talking true Gregorian chant, that's another animal. Maybe someone would know better than I, but I don't believe much of what we hear are "scales". At least not any that I've known. Also the layout of the "songs" tend to be unstructured and difficult to internalize.
That and if you notice, most chant sounds as if they cloned a monk from 1240 AD and reproduced him to record CDs for us. They all pretty much sing with the same voice and same inflection, no single singer distinguishable from the others. Vibrato is pretty much verbotten on many pieces. Bob the opera singer standing next to Top 40 Tom would cause both to have a bit of re-adjusting to do.
I guess my point to borncatholic was that I don't see a pastor in the average parish being able to announce "hey we're going to Gregorian chant... any volunteers?"
I pinged pyro, he sings the chant. He'd probably have some input on the subject.