Lee surpassing Gai is a big and valid possibility.
-Quote from databook:
"The moment that
Lee surpasses his teacher might be the moment Gai reaches the end of his Ninja way!!"
-Successors surpassing their predecessors is a main and common theme in the manga.
-Gai's goal is to raise Lee to be a great ninja:
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-Gai was hinted to be passing the torch by having troubles in keeping up with His student:
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-Quote from databook regarding to Lee:
" He works towards reaching and possibly
surpassing his master. But when it comes to vigour and eagerness, he even surpasses Gai! When the two of them are together, the word "impossible" just doesn't exist any more!"
Edit:
- Also Kakashi is surprised that Lee can open 5 gates at young age, if Gai was above or equal to Lee at the same age he would not be surprised:
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