We don't know when Gillian Flynn's new novel will come out. There are so many factors that could influence the release. If she's just started writing, it'll take time to complete, then there's the editing and proofreading. And publishers also need to plan for the best time to launch it in the market. Without any news from her or her publisher, we're left in the dark about the release date.
Since there's been no official word on whether she's even writing a new novel, it's impossible to say when it would be released. Gillian Flynn might be in the early stages of conceptualizing a story, or she could be waiting for the right inspiration. Also, the publishing process can be long, involving drafting, editing, and marketing. So until there's an announcement, we can't predict a release date.