The first waterfall that gives access is Þröskuldur http://www.isalp.is/problem/throskuldur (Do you have a photo?)
The route you are describing could be Nefbrjótur http://www.isalp.is/problem/nefbrjoturinn but the description sounds more like it is located before you go up Þröskuldur.
I think your route might be a first accent based on what is logged on the website.
The website doesn’t have a lot of photos from Grænafjallsgljúfur, can you maybe send me some?