This is an interesting topic.....Well, I guess the best way to put it is that people in general understand that technically according to the anime and manga the Opening of the 8 Gates isn’t a Kekkei Genkai. However, in this game it will be classified as a Kekkei Genkai:
1) It is obvious that it takes hard work to accomplish the task of opening the 8 gates. Learning it at any specified level isn’t possible. For example, Sasuke’s body had a hard enough time copying Lee’s taijutsu, and that was with the help of his sharingan.
2) The reason for having it as a kekkei genkai is because Rock Lee truly began learning how to do the technique at a young age. He was still an academy student (and considered an academy student with no skills, henceforth the label dropout) when Guy/Gai met him. Lee wasn’t his student or on Guy’s team during that time. However they both had a connection and it is obvious that he trained with Guy from that point forward. It’s no surprise that he was placed in Guy’s Team. The training he did with Guy during his academy days, were the foundation of his learning how to open the eight gates. Neji and Tenten’s bodies could not handle the rigorous training that Guy put them through, but because Lee had worked with Guy as an academy student before officially being on Team Guy he was not only handling Guy’s initial training with Neji and Tenten but also was learning more techniques (The Lotus and the Opening of 8 Gates) while his teammates where going home from a hard day of training.
3) Because his true foundation of learning this technique was during his academy years, it’s only feasible to classify it to where a person has to obtain the foundations of the technique as an academy student. The only way we start over in the game is through kekkei genkai.
4) With this technique being a kekkei genkai, in the truest sense we are stripping away everything (Genjutsu and Ninjutsu) and concentrating on only Taijutsu like Lee and Guy.
Did Guy teach Lee the Opening of the 8th Gates? Of course, no one can question that; however it wasn't something that could just be learned by Rock Lee at any point of time. Because of his earlier training with Guy, as an academy student and up, Guy knew that for the most part Lee's body could take the rigorous effects of the technique.
"You can't teach an old dog new tricks." Kids are able to learn new languages and techniques at a young age much easier than an adult. Because adults are content in doing things their own way and don't desire change."