It's probably the “relaxing spirituality” aspect that causes Gregorian Chant to be associated with New Age music, but the two genres have another point in common as well. That's the use of a “free time” structure, where the music does not have a definable time signature. Time signatures probably had their origins in work music. For instance, rowers on a galley would sing to coordinate their rowing actions. Most music before the Renaissance, though, was in free time, and most types of “world music” are as well. New Age music probably borrowed the use of free time from two sources- free jazz, and world music. But the use of free time in Gregorian Chant still gives it a similar “feel” to New age music. Many people also find it very suitable for use while meditating, as long as they aren't worried about the explicitly Catholic nature of the chants themselves!