A press release for HBO Max said that Gossip Girl will be available "within the first year" of the site's launch. So expect it by May 2021 at the latest. For now, the series is still streaming on Netflix for those who need their GG needs satiated ASAP. The show likely needs to run out its Netflix contract before it will move over to HBO Max, which could happen anytime in the next few months to a year. So far HBO Max has only released their June 2020 schedule and Gossip Girl is not part of that slate.