Slightly TT, so spoilerized, and I apologize if anyone is offended.
The Grigori were the Watcher angels, who watched over mankind in the early days. They supposedly fell to temptation of human women. They stopped Watching, and did all sorts of things, such as bringing fire, tools, alcohol, cosmetics, etc, etc, to mankind. And they fathered children with human women, and their children were Nephilim.
From what I read ages ago, they were cursed to walk the Earth until the end of time, forced to 'watch' humanity, and await judgement in the end of days.
I remember some theories on the subject that said the Grigori, and the story of them fathering the Nephilim and giving knowledge to humanity, was an attempt by the Church to explain some of the older religions in a Biblical fashion. Instead of Prometheus bringing fire to the people, it was angels Gadriel brining make-up to humanity. Similar to how the Church apparently created a list of demons that corresponded with the old world gods.
Who do you mean when you say "the Church?"
IIRC, there is no direct Christian canonical reference to the Grigori, at least by name. The Grigori were supposed to watch over Man and, I think, educate him in ways to keep him close to God. Remember that "watch" may mean more than to merely observe.
The reference to the Nephilim was in Genesis, part of the Christian Old Testament
and the Jewish Torah. The 5 books of the Old Testament were not altered (except by varying qualities of translation).
Saying that "the Church" did this or that is not really possible in this context.
The biggest source of angelic lore is probably the book of Enoch, which has been dropped from Jewish canon and mostly ignored by Christianity.
Edit:
There is a bare mention of a Watcher in the book of Daniel. But no details as to what a Watcher is/was.
