I like the flow of the story and the way it fits canon so well I had the feeling I was reading a missing scene (except for the hot smut:).
What I like best though, is your characterisation of Gellert - I could see The Next Dark Lord In The Making form the beginning to the end of the fic, with his sociopathic thinking and behaviour, in contrast to what we see of Albus; yet he is still very different from young Tom Riddle - with his need for Albus and his idea that the two of them together would be better than just himself alone. And that, imo is the main difference between the two, and why Gellert in his final moments is able to feel remorse, and Voldemort isn't. Perhaps not love, but still "something".
p.s. sorry for the anonymous comment, but I don't have an IJ.