I make no claim about it; I haven't tried it myself, and I need to find some reliable source who has, before I download it. Call me neurotic about downloading .exe files. I do suspect it's totally on the level, though, after reading the FAQ What does the program Grégoire cost ?:
Developing the program was a considerable amount of work. To give you an impression: the complete application is compiled from over 45000 lines of code (almost 700 pages!). You might expect that such a program is sold for a very high price… but that's not the case! The program has been developed and deployed in a spirit of charity, and it will remain available along the same line, contributing to the dissemination of the vast patrimony of Gregorian chant. Today we consider it abnormal that a regular user would not make any contribution. Therefore we ask you, after you've tried the program for one month and when you wish to continue using it, to donate a minimum amount of €50 (about $50 or £35) to a charitable institution of your choice.
Hackers don't, in my experience, usually request a donation to charity. So I suspect the thing's OK. But if I can't find a reliable recommendation, I'll have to partition my hard drive or something, to try it out. Will re-post in that event.