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the power increases 100 fold, so if A is 50, in the 8th gate his power would be 500 and if B is 30 then his power will be 300 with the 8th gate opened.
so it really depends on how much Lee would achieve in base, and would he surpass his master in base strength without using the gates or not, but logic in Narutoverse suggests that the newer generation is stronger than the older generation, so there is a high probability that Lee would surpass Gai, and will be able to use other techniques(more powerful than Night Gai), but its the manga so it could go both ways.