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You are not asking the right question, but we are here to help :) The problem is not with carrying it on board. The problem is Japan is one of the most restrictive countries bringing medicines sharing this top tier with the United Arab Emirates and maybe there are a few more countries but I can't recall any other that strict. This is the page to read. While some medicines require obtaining a permit ("Yakkan Shoumei") before you leave (did I mention they are strict??), for your case the quote is this:
When you bring your personal medicines into Japan up to the amount described below, you do not have to apply for "Yakkan Shoumei", instead the officers at customs check the amount of your bringing medicines, and then you can bring them into Japan.
So you must declare your gel at the border and not carry more than two months supply. To be on the safe side, not more than one, in case I misunderstand which category it falls under. One tube is safe. Make sure to carry the original box and paper it came with although diclofenac should be familiar to them but let's not count on it.