gai can use ninjutsu(he has summoning jutsu) just he is a taijutsu style fighter. He simply trains taijutsu.
Lee "cant" use ninjutsu. This is because he simply has no talent in ninjutsu at all.
Have you ever been around a person, who just has no talent in something at all? Like a guy who tries to play soccer, who has zero foot coordination? Or tennis with someone who just cant hit the ball?
Well thats lee with ninjutsu. He technically can use ninjutsu(as in he doesnt have any bodily restrictions to stop him from useing it, just he has no talent in it at all.)