According to the wiki, Gai is able to use ninjutsu and genjutsu, so why doesn't he use it?
I'd say because they aren't his strengths - clearly Taijutsu is so why should he use something that he isn't great at?
It could also be that he has used them in the past and it, unfortunately, got him into some difficulty so he had to fall back on the tried and tested techniques of his Taijutsu :)
Gai is based off of Bruce Lee, and Bruce Lee was known for his extraordinary fighting skills (taijutsu in this case). Furthermore, Gai is known as the best taijutsu shinobi in Konoha.
Thus, it would only make sense to accentuate Gai's physical prowess and focus less on genjutsu / ninjutsu.