I don't know of any youtube videos but I might be able to talk you through it.
First some questions: Does Gabby know backward circles? Can she back up? Back up towards you? Pivot on a perch?
I've taught backward weaving 3 ways. I prefer teaching it after the dog already knows how to do a bckward circle in both directions. Weaving backwards just becomes asking for a circle one direction but puting out your leg and interrupting it half way through. Then put out your other leg and ask for the circle in the other direction. Basically it's a half circle right followed by a half circle left. Many dogs I've taught has learned this extremely quickly.
The other way I've taught it was before one of my dogs knew how to do a backwards circle, but he did know how to reverse towards me and was comfortable standing between my legs. What I did with him was change the direction he was backing up. Instead of facing forward with my legs open as he was used to, I put one of my legs behind me and worked on him getting used to backing up (which he already knew how to do towards me) to me side ways. I practiced in both directions. Then since he was stopping when he got between my legs I lured by pushing a treat into him until he was now past my leg an then asked for the other direction.
A third way I've taught using a wall and a dog's prior knowledge of backing up in heel. This time as the dog was backing up next to me (on my left) I swung my leg out behind him and pushed him through with a treat.
here's a video of the 3rd method:
Note method 2 &3 are very similar, but 2 is the dog thinking a bit more, 3 is obvious luring. With both methods it takes the dog longer to learn how to do a TRUE backwards weave where the dog is the one making the transitions from half right circle to half left. In method 3 it starts out as the human doing more of the moving and the dog is doing a straight line backwards.