It looks like Discord stores unlimited amount of history like Gitter but it has more features. It's definitely a downside that you need to be registered in Discord for read-only purposes and your discussions are not indexes by Google. That said, we do have mailing lists where you don't need to be registered to view messages.
I think it would make sense to start a Discord server for JanusGraph and see how much popularity it gets. I guess we shouldn't abandon Gitter just now because we have 864 people there but we should prioritize using Discord and wait until it gets the same number of users as Gitter has right now.
As for another aspect - I'm also in favor of using Dedicated server because we will be able to create multiple channels which should be very useful to structure future discussions. 
I didn't research much but if there is a good way of redirecting users with Gremlin questions to TinkerPop server and JanusGraph questions to JanusGraph server than it would be great. I would imagine some dual-linking between JanusGraph server and TinkerPop server but not sure it that's possible.