We introduced Gitter Topics in September 2016 as an experiment. Unfortunately, it never gained traction and remained in beta through its life-span. After some consideration, we've decided to deprecate and remove this underused feature, allowing us to focus on the chat application.
Topics will be completely removed from the Gitter UI on September 1st, 2018 (2018-09-01). We will disable the ability to create new topics/replies in the week prior.
You can export and download all of your topics by visiting your topics home and click the "Download topics" button at the top. The download endpoint is rate-limited to once per hour and is only available to community admins. The format is ndjson (newline delimited JSON) and you can parse each line as it's own JSON document.
GitLab issues offers all of the same functionality and a lot more (full-disclosure: Gitter was acquired by GitLab). There isn't a direct way to import your Gitter topics but you can use the GitLab API to create issues from the topics data.