If you are using the Dissenter browser or browser extension, your profile already has a chatroom. Just go to your profile, open Dissenter, and select Live Chat. That's your public chat room.
Dissenter chat message persistence is 7 days and requires a Gab.com account. If you do not have a Gab.com account you don't know what I'm talking about.
Those are not DMs. That is a public chat room. DMs will be entirely something else native within Gab Social.