I don't know how it was between Gaby and her mum but talking about personal experience here, you can sometimes teach things to your own kids you can't to other, the opposite being true too though. When a kid relies solely on external training, he/she won't be able to get the same amount of time dedicated to him/her than a coach/parent who is constantly present on the ice infusing advice on a day to day basis. Again, I don't know that it was like this for Gaby and her mum but I do recall an interview where one of them mentioned the role of mum/coach being fused at times. If done in a smart way, your kid can really benefit from those daily tips and constant reminders. I taught piano to my kids and the fact that I hear them play every day and can intervene to help them out whenever, is an advantage compared to a kid who has only his teacher for a couple of hours a week. Of course, there is a danger of being overbearing to the point where the kid opposes resistance. That is where Gaby's mum must have been a great coach knowing when to step away and intervene. I also don't want to imply she wouldn't do marvel with other kids just maybe not to the same degree than with Gaby and Guillaume due to their special circumstances.
I also remember Mark Hanretty being the only commentators raving, while commenting their SD performance at World 2016 (the waltz), about the role Gaby's mum played to help them develop their great basics.