I count three disputable points that stand between you and success on this one.
Rightlydivide got one. Who’s to say the Eucharist is something that could make the Gates of Hell prevail? I’ve heard a number of variations on this one, and to this point, I haven’t heard any good reason to name a particular thing that would cause this to happen. I could just as easily make an argument against the identity of the CC as the One True Church by saying “Look at how many Christians aren’t Catholic. If the Reformation really were breaking up the OTC, that means the Gates of Hell prevailed. Therefore, the CC cannot be the OTC.” (Look at me, demonstrating that I understand the form of the argument). In that case, all you have to do is assume that the Reformation doesn’t constitute an occasion for the Gates of Hell to prevail.
Here’s an interesting thought, though: If you were a Christian in the first century, how would you have assessed the Reformation as a hypothetical?
Number two is the accusation of blasphemy. I know some anti-Catholics do it, but there’s a reason most of us don’t. Even though we agree in denying the RP, most of us don’t accuse you (or the EO or Lutherans or Anglicans) of blasphemy. It’s more than courtesy, and it’s not like we’re quietly judging you without telling you about it. Unless non-Catholics feel that Catholics have erred so grievously that they can’t be called “a Christian church” anymore (which does happen on occasion), we don’t accuse you of blasphemy because we really don’t think it’s right to follow the logic there from a denial of transubstantiation. And without a proper accusation of blasphemy (which I would not have predicted you’d be fishing for today), there’s no case for the Gates of Hell hypothetically prevailing.
Third, the conclusion that the Gates of Hell have prevailed (when the CC fails to prevail) is dependent on an implicit faith in the historic reality of apostolic succession and the responsibility of the CC to safeguard the truth. This allows you to equate a failure of the CC with a failure of Christ’s Church in the general sense, and non-Catholics don’t equate things that way for the CC at any point in history.
It’s a fun game, though. Your logic does achieve internal validity if you can get non-Catholics to agree that the CC was at one time synonymous with Christ’s One True Church, the doctrine of the Real Presence and/or transubstantiation qualifies as blasphemy, and that this would qualify as something that causes the Gates of Hell to prevail against Christ’s One True Church- which, as we already established (how did that happen again?) is the CC.
Good luck with that.