I am looking mostly for the essential information on polypohony, gregorian chant, techniques of the time and their development etc, in order to get a grasp of the subject, names and times so I can later on focus my study on books.
Here is an example of a documentary which is readily available, depending on where in the world you are located, and which appears to have some relevance to what you want:
If you want to hear some examles, there is a whole playlist of short performances here, which gives you a lot of names, but no discussion:
You might also be interested in some of the (free) courses offered by Coursera. For example:
You might need to register with the site to get access to this, and probably only week one of the course is relevant (A brief history of notation), but I have found the several lectures in week 1 of the course very informative.
I will have to leave it to you to decide if any of these are "good" ...