as has been explained several times gai can use ninjutsu. according to the data book he is a 3. he just was inspired by lee and specializes in taijutsu becuase as has been shown by naruto and j-man that the students way of the ninja is also the teachers ie show that lee can be a splendid ninja using only taijutsu.
link above is for gai(look under abilities it shows his ninjutsu is a 3)
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link above is for gai(look under abilities it shows his ninjutsu is a 3)