So far (and I have just started with this), I have found that ?rel=0 works when you insert a video in any page - with the exception of the video gallery facility (which is rather a shame).
When I Google this I notice that people using other web template solutions (Wordpress etc) sometimes have a similar problem. Some have suggested that something in the gallery code overrides the modified URL. I don't code so I don't know but in at least one of the Google links I read, users indicated that when you insert a URL and add ?rel=0, the gallery verifies and modifies the url to correct it and thereby enable the adverts. I don't code - but I guess you can look at the underlying code and see if it matches exactly the URL you have entered in the gallery - or if it has been modified.
In fairness, YouTube is free for us to use and the revenue to operate comes from advertising. Perhaps there be a 'code of practice' operated by template providers that they will enable advertising in a gallery. I don;t know, but whatever the reason, ?rel=0 seems to work fine on any page in my site - with the exception of the video gallery. So the answer maybe to create a video page without using the gallery function.
[i am not an expert and I don't do coding. I am just sharing my experience so far]